Alex's Caves It adds truly amazing cave biomes. Like seriously check this mod out!
Born in chaos It adds new undead/spirit mobs.
Aquamirae It adds new mobs to the frozen ocean biome.
The Dawn Era I just love dinosaurs, rawr!

I put them in order of how much i think they fit in this modpack. The highest being a perfect fit and the lowest not so much.
Cheers for reading my suggestions! 💯