A new way to get to the nether, working exactly other cave biomes, needing a map to be found, but being a nether corrupted biome (nether veins, a living biome).
This biome would have a guaranteed structure, that would be a hole that leads to the nether (tp players that get to the end) (no portal)

Possible features / mobs:

• Pigman tumor - a blob of pigmans that were fused together when they traversed the portal, this aberration of nature will have half the strength of a warden, but the same health, on death will drop a infected core.
• Nether Vein Guardian (small ghasts) these will spawn naturally in the biome, they're the protectors of the nether veins, faster and tougher than regular ghasts but only shoot fire like blazes do. think of a armored ghast, with cracked netherrack around its skin. They will ignore pigman tumour even if attacked by one.
  On death they drop their armor, living plate (netherrack) - these can be used to make a new living armor, together with infected cores, the full set will make you invisible to nether vein guardians, and will give damage bonus when on fire, and some fire resistance.
• The nether veins will be a giant corruped cave, that has netherrack veins all around it, the water is red and poisons on contact (may be picked by a bucket), together with overgrown crimson and warped fungus mushrooms (bigger then regular trees found in nether), and a pulsating netherrack block (so the whole place beats like a heart, may have a sound effect too)
• Before being able to enter the hole to the nether you will have to defeat a boss that guards the hole, behaving like a cover that cannot be trespassed by regular methods, (maybe he generates the hole on death, so its easier to prevent exploits idk, but make him unable to move by walking) i thought about a thicker version of the end worm, but this one ate trough the bedrock that separates both worlds, and will attack the player by digging and rising from underground knocking players up, since this monstrocity can eat bedrock, his teeth are the strongest weapon of the game, even with full netherite armor, a bite will be almost one shot, he will also attack by spitting lava (no blocks, but visually like a flamethrower), the boss will have netherite platings around it, and will be resistent to fire, lava, and getting too close will set players on fire, he will increase guardian spawn rate when provoked, however just like another mob from the nether, this creature will be weakened with water, getting in contact with splash water potions (eventually) and water blocks (faster) will turn him into an obsidian version, (only when low on health) he will become tougher (second phase) but will also be unable to move as he did before (because his body solidified), in this phase will be immune to projectiles, but will no longer burn players that are nearby, and will change the fire spitting and digging moves for regular bites.
  On death this boss will drop netherite scraps (the ones that you use to make ingots) and a molten core, this will be an upgrade that can be added to the pickaxe (with the smithing upgrade) that will turn a regular netherite pickaxe into a molden version, that will be able to break bedrock (or work like the interdimensional pickaxe and take you to the nether by digging down, idk)

To get back from the nether you can enter the hole on the ceiling that will be above you, you will be teleported to the cave you were before.

That's the idea, some interesting extra mobs could be added, like a smaller version of the boss, as some annoying worms that will show up when digging trough the nether, some other terrestrial mob on the caves, but i haven't really thought about it that much.
Got this idea from Yung's better portals, but thats an old discontinued mod and the dev seems to not want to update it, i thought the idea could fit with your cave mod so there it is.

Pretty sure it was mentioned before in his server by one of the mod dev that theyre not interested with vanilla dimension themed cave. I think they want all cave biome to be fully unique rather than just a vanilla biome with a twist,

And sadly is that Alex are know to be very rarely doing people suggestion. As he have his own plans for his mods

No. Nether Caves would just be a Nether biome + this isn't the place for suggestions

Well, that was not a very delicate answer, where am i supposed to make suggestions if your discord link doesn't work? lol
Didn't see anywhere written that this was not a place for suggestions + fix the link

The only discord link that work currently can be found on Alexs Mobs mod page at the bottom of the page theres a link for it