As I promised, and as we agreed, I write the suggestions here.

First and foremost, obviously, it could add recipes to Crimson Cult armors and weapons and perhaps even banners. Or, alternatively, items based off those designs and which are close to them/replicas. You can take inspiration for recipes from Tainted Magic addon for TC4. Requires only to make the recipes and probably a new aspect.

And now to unique things. Crimson cult content may focus on either warfare aspect of things, or Cult aspect. Or both.
Generally, it may as well have more unique enemies added, establishing a few lore bits among already existing things.
For enemies:

1. First addition would be the addition of the enemies, which are not implemented yet. Remember the other models made for cult, for ex. one with an 8 barreled pistol, or the crossbow guy. Perhaps, Cadaver type zombies (inhabited zombies) can have it's equal in the fact, that lesser "troops" could have the same armor as on the Cadavers. May as well make this armor obtainable. Can be made by creating special spells, and cloning default skeleton code while doing some adjustments.
2. Restoring Praetor's naming and skill convention, just like with other bosses done for TA. Maybe giving it some ability as well. Possible to do by digging some older code from TC4(?).
3. Adding purple-colored sets, as a "Royal", rarer division or one which encountered in some specific places. Have better stats as well. Done by copy-pasting it with other RGB colour and different stats.
4. Adding brand new enemies. One is the Eldritch Construct-like towering mob in armor, with either bigger Cultist in it, or captured and repurposed Construct. Next is the some sort of siege weapon, or a "tank" of some sorts. Essentially, being a construction on wheels and composed of gears, wood, and metal plates. And maybe a cult member in light armor, which has the crossbow turret manned, both with a shield and without. And, finally, the Pyromancer - being the cultist in improvised gasmask, and "flamethrower" about which I will tell later. And a shield-bearer, or the unit which carries the shield, rarely with runic shielding. The rest of unique enemies could be thought-out later, but among them there could perhaps be an autocaster, colored and repurposed to fire Cleric projectiles.
   Most of those things can be re-used from existing code. Others are done by manipulating current spell mechanics, and shield - by creating some special objectiles for AI to use shield, which will make it immune to direct damage from the front of it's hitbox.
5. Some sort of cultist healer won't hurt. Or adding the special varient of the cleric, which will attack allies but with healing spells. If alone, will resort to usual cleric arsenal.

Other things:

1. There could be a research or ability to learn to cast the projectiles which crimson Clerics use. And later mount those on your autocasters. I'm pretty sure this can be borrowed from the code of the Cleric and put into the "common" use.
2. Return of the Eldritch Obelisks could be performed, however they will not transport player to the Outer Lands, or anywhere really. Instead, they could serve as the role of monster spawners, and lore bit could add that those obelisks, according to TA, did have some "nodes" present as in "ancient" thaumaturgy. They will behave exactly the same way as in the TC4, some spawning empty and some with 4 clerics linked and stabilizing the special permanent portal. Otherwise will have chance to spawn Eldritch Guardians.
   It won't transport players to OL, but rather... a "special" dimension or room, where it acts like a vault of some sorts. With limited space and unbreakable walls so you can't get out. And surrounded by bedrock and void.
   You can modify it though, and make it so that obelisks are not floating, but rather cracked and fallen into pieces most of the time. Balancing may be looked into later.
   Alternatively, obelisks could allow player access to Cult dimension of some sorts after heavy research.
   Can be done by rummaging thru the TC4 code.
3. Player, after enough research, could create and use similar portals which generate natually (Crimson portals). They can be used for both item transportation like mirrors, and for player teleportation around the place, and across some dimensions by building or applying enough resource to open the portal to required place. However, it will have it's own big risks: like in Eldritch Obelisks, players will recieve a HEFTY amount of Warp when going trough the portal due to not being as insane as Cultists, and not prepared to see the horrors of such transition, and if portal fails or has not enough energy or stabilisation, will kill the player and spawn Angry Zombie instead. For items - it will destroy them, if used for item transportation and same thing occurs. Will also explode.
   I'm sure some code can be re-used.
4. Some ways of reducing Warp and resisting it based on the research, and perhaps even adding VERY COSTLY, and EXTREMELY dangerous craft item, which could remove some percent of the Perma warp, but very little at a time, so it will take a lot of effort to get rid of.
   Can be done just via value change and crafting recipes.
5. Some new weapons could be introduced, like a counterpart of the Pulse cannon, using Crimson-cult unique energies. And mentioned flamethrower, which uses either essentia to create magical flames, or raw ignis crystals mixed with aer to produce fire.
   I'm sure this can be done by re-using spell mechanics, modifying it, and creating custom weapon.
6. Crimson Cult branded Casting gaunltet and Matrix. Obviously. Could allow to use Crimson Cult raw energies they use, or combine them with regular thaumaturgy.
   Can be done via copy of the default gauntlet, and changing it's attributes. Could perhaps also result for the Matrix.
7. Insanity bonus - if players want so, they can make it so that with certain item or rituals, it will transform their warp levels into buffs.
8. "Blessings from the Dark Gods", essentially perforing certain given tasks for blessings, often sacrifices or giving tribute. Can be done like a Matrix ritual in part, and have the requirement for certian thing randomly generate on a parchment. However, each such ritual has a sizeable chance to contain one negative aspect or curse.
   Up to being able to summon the avatar of one of the Gods or the entire pantheon (can be inspired by the Eldritch guardians and Warden look-wise, and be somewhat half-transparent, other looks and colors will depend on the God) which will assist you until time runs out or it is killed. Sometimes, they could require player's body, which will in turn, during ritual or end of it, kill the player and will obey the ownership. Has a small chance to turn against the player.
   Perhaps a new enemy or mob too, which is powerful enough. Be aware that those could serve as bosses too. Will cause some crimson cultists to flee?
   I'm sure this can be done via copy of the Matrix ritual, mob summoning, and potion effects+negative effects. Need to be discussed in detail later.
   Could require making some statues of creatures which resemble the "Gods". Which, according to style of TC, may not be gods but in fact sentient mass of Primordial energy or so.
9. Sacrifice and Ritual mechanic which uses blood and sacrifice for certain things required there to craft. Can be tied in to previous suggestion.
10. A change, but some use for the "Opening the eye" ritual to mean the summoning of the Gods into the realm of Overworld to rip the tear in reality to invade. Which will require so much energy, that they alone can't really do it nor player in their capacity. Nor even if Gods are brought to Overworld - still not enough power to make a rift. Just a textual change.
11. A special focus or item, perhaps, found or having key research bit in the Dimension could perhaps allow player to summon or create their own mini-army of the Cultist-like mobs. After enough research, of course, and killing enough "pure" cultists and their types. Either the same cultist members, or their astral-energetic projections, where there is no texture and it is just the humanoid transparent model, which has the armor, stats, weapons and may as well shine as if enchanted.
    Done via graphical effect manipulation, and summoning of the "pet" mobs (borrowed from wolf or other combat tamed mobs) with the ownership to the player. Can have a goal or task to despawn after certain time, or just appear and remain until killed.

The dimension.

1. Could resemble the almost same world as overworld, but warped in places.
   Some of the terrain will look as if it was warped and corrupted heavily, sometimes create impossible shapes, geometry and structures, and etc. Can be done like Emptiness, with some of the "structure" generation.
2. Will largely have red-ish skybox with stars and red sun? Perhaps addition of the same cycle. Sun could look like a giant eye or tear in the fabric of reality, which causes the illusion as if it always watches you.
   Can be done by recoloring the skybox and adding more skybox features, will require art work.
3. Could have it's own food, plants, angriculture and food, if player wants - he could live there.
   Can be copied and retextured from vanilla.
4. Dimension would slowly afflict more Warp onto the player, so some sort of the Sanity protection from previous part is required.
   Can be done via dimension check and time spent in dimension.
5. Structure generation. Could be very simple, and somewhat randomish. It will range from simple war tents (built) with spawners and pre-spawned cultists and some chests with loot and occasional wood pole targets (for target practice) and watchtower; watchtower with ranged mobs on top always, has some loot;agricultiral small farms; library building with clerics and lot of knowledge based items; Eldritch-like obelisks, which mirror the ones in the overworld and perhaps can be used to get into dimension, and out on the other side like rifts. And three (?) main structures like Spire in the Emptiness: Big Fortress, Cathedral, and perhaps something unique, I don't have more ideas on that. Will pretty much have same mechanics of the Spire/Tower, likely will contain Royal Cultists alongside the regular ones. All of the structures, where appropriate, could contain Crimson autocasters.
   I think the generation of all of that could be done by copying the generation of the Emptiness, except there will be a need to build the structures, and create the copy of possible doors and keys to them which resemble metallic gates.
   I can help with structure building if we get to it.
6. It could, if possible, have the separation in biomes like you did with Emtiness, and each biome will be dedicated to it's own God in style, e.g War, Excess, Rotting, and possibly others or completely different concepts. Also on this, certain elements of the strctures could be interchangeable. Like Brass and predominantly red bricks in the area of God 1, but other colors and decor in Area of 2. Changeable via schematic block type replacement of only certain blocks, and maybe unique boss spawns depending on the biome too, same with some enemies or their coloration.

Purpose of visiting the dimension: gaining more important research knowledge, more unique items which are obtained only there, and possibly learn to use the "God avatar summoning" for yourself to use and expand your Sacrifice and Ritual abilities, and of course to challenge yourself. And fill-in the gap for the Cult which is currently empty. Also will explain why they are so zelaously insane.

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So far, it is the first draft of the idea, it can be changed later during development and some features could be elaborated and redone as well. It took me a while to write this long-lasting concept I had for a year now, hope this reading wasn't too tedious. :D
I had a mini-suggestion about taint content, but it is far smaller and has way less things to do. I can list it as a separate issue if you wish me to explain it.

I had similar thoughts when I thought about what else could be added to thaumcraft 6. I had thoughts of a special chapter that would talk about the knowledge of the crimson cult and its world (not the void). To open this chapter, you need to get a special book, which is a rare drop (25%) from the boss, which can be found in rare generation, to access which you need a crimson blade. The boss is called Giant Crimson Portal, it spawns elite Crimson Cult Crusaders who will have their own armor, Crimson Void Falchions (slightly better than a void metal sword, but worse than a Crimson blade (Crimson blade is buffed), but falchion causes deceleration), and rarely shoot entropy balls.
Then you can use this generation to get into the world of the crimson cult
In the world of the crimson cult, there is always a night with a blood moon, there are rarely lakes filled with blood (or red water), there are tainted biomes, but the most common biome is a gray desert in which there are Gothic castles in which you can find a way back to the overworld, pools with cleaning liquid, large libraries, and even expensive items that need to be scanned for new research. In the tainted biome, you can find the ruins of the ancients.
In the chapter of crimson rituals there are studies of crimson forging, blood alchemy, crimson auromancy, this chapter expands a little more on the other chapters of thaumonomicon.
There are still a lot of things that can be added to this chapter, but some of them you have already written, and others I will write later.
It is easier to make my own thaumcraft update with blackjack and hookers than wait thaе someone implements all my ideas for me