Just some Ideas:
Make some kind of boss "vengeance spirit" something to upgrade your mod to something more big, its made by the "spirit's crystalized will" (Realy hard to get without the correct weapons).

Make a kind of blooded magic thing (not like bloodmagic) but more like Thaumcraft or Ars Magica 2, with the blood as the main base or element.

So you will use the vengeance spirits to strengthen your spirit, and use it to the magic, its don't need -and can't be because its gonna just suck- to be something like "ohh I made a item now I can shoot fire balls everywere" make it more mystic and realy hard to do.

And it can be like, when you get the "knowledge to use the spirits will as magic essence"
you start to resarch more about it and discover that the blood is more powerful than you think(why?? because blood is the more easy way to conect the physic world with the spirit world its other thing that you can use to have even more stuff), and you can use it to make powerful tools and armor that uses your enemies blood to repair itself "its like mending but more cool and a pasive ability" its easy to think in this way.
But its is not just blood is the blood with the Spirit's will, (so you can make high moded tools or tools that can be enchanted by the spirit's like:
you have a lot of "spirit bat crystalized will" so you can put it on your boots to make then have the feather fall buff, so you can make combined will (another late game thing) so you can combine the bat and the chicken will so you can make the "flying entities will" to make you fly.
(remember when you put the blood in the world and it just become hardened blood, use this concept to make the tools and weapons) the weapons will be in other coment I don't know if you will even read this one.

So your mod will have two main things:
The blood original concept of the mod.
The spirit's will, some king of next step to the mod.

Its something close to:

"Using the spirit's will, I discorvered many things, good and bad ones.
I discorvered that I can fuse my spirit with the other spirit's will and this let me use some kind of magic (because its not just magic its something more for history and things to add purpose) I don't know if this can be used with the actual magic in the word (just a place to have conected mods,) but I discovered that the spirit's have a conection among themselves, so they will be realy pissed about you if you have "the crystalized bat's will" close to a chicken (remember they are conected like flying ones, aquatic ones, nether ones) and with something more big,
I din't discovered much about it but It appears to be another dimension (another place to have even more stuff), its not just a another dimension but a dimension that you can't enter with your physic body, only your soul/spirit (kinda cool right ?)
And they don't like when I use they friend's will, (that mean if you use to much spirit will this will spawn a lot of spirits to kill you)

and change the sprit atack to pure, because they don´t atack your body they atack your -soul- spirit

Other enemie:
Crystalized Vengeance spirit:

Its like a vengeance spirit but can be killed with a sword because hi's will is so strong that him don't gave up even before death so him's spirit somehow crystalized it self (but not enough to have much physical damage), but because its a spirit so his body is not weak against swords ou physical attacks, and have pure damage (your armor can't protect you) so the item that trade the damage taken by the spirits for hunger gonna be more useful,

Status:(Same as the spirit mob) but with a 20% more status and a rare drop (to the vengeance spirit boss), and only receive 20% or lower physical damage.

change anything that you don't think that can be in the mod, the mod is yours so I'm just pointing some Ideas.

now about the weapons:
Is not like they are just a blooded pickaxe or sword, nope, is something that is just cool is like this.

"When I fused the blood with my spirit, I discovered that I can control the blood somehow for now I just can make a bit of the blood levitate in the air for a few seconds its strange but I feel that I can use it for more.
Alot of resarch after
So I researched more about it I discovered that the blood is more easy to control with A more powerful spirit will, I don't tink that I can strengthen my spirit normally, but I tink that with the spirit's will that I discovered about it can solve this problem (this is after the spirit will discorvery, because its too late game the weapons is not something physical is your power to control the blood to strengthen your tools so you can use it to have your mods in all modpacks with this unique idea, (like the matter overdrive I love this mod) so you will fuse the tool to have this linked with your spirit and use it to power the tool (its like rf but made with blood and can be used with all tools),I discovered that I can't have everything linked with my spirit (because its gonna be too op, its like a nerf and a way to make you go to kill the "VENGEANCE SPIRIT BOSS")I discovered that him have the knowledge to have the spirit linked with "The great mother" (is the name of the dimension that I said before) (can change it if you want, but I chose this name because is from another Idea), I discovered that the great mother is conected with the Spirit tree, I didn't discoreved the name of the tree I'll find it latter now I just want to finish with the weapons. ("Yggdrasil") (is from another thing)
so with this you can have a lot of weapons and amor linked with your spirit, not everything and this gonna cost alot of spirit will because of nerf purposes, so will be like you NEED to kill the vengeance spirit boss to have the weapons.

so you have the:
Linked blood (is not like normal blood and can't be used in the same way,) is blood linked to your spirit and is used to strengthen your weapons and tools and armors too, but can't be used to much because this can kill you, because your spirit can disapear.

just a silly thing, about the dimension you can't carry any item in it (you are a spirit) to go there you need to separate you spirit of your body (I don't know something to put in here, put a talisman or something) Is a place to you purify your spirit and apologize about the spirit fusing thing, because is just to horrible what you did, (Evilcraft is not a name for good magic or something so I came up to it), so you can have even more stuff (I stop if you want, is a lot of things and this can start to be annoying)

And if you can spare some time of yours to say what can and what can't be in the mod to me refine my ideas.

man I gonna be realy happy if one or two of my ideas can be in the mod, because this will be realy cool (like the weapons one is one of I'm want the most).