Because you're soliciting general ideas:
- player pred is nice, and could be used as a source of mana for players that want to be prey less often
- a small number of recurring NPCs with whom relationships are built
- rival who (in combat?) can vore or be vored, treating player differently based on how they've been treated
- upgrade vendor wizard who vores the player, extracting "payment" (experience from enemies?) and using it to enchant the player
- ally character with out-of-combat vore scenes
- implement checkpoints/warppoints as creature or NPC with healing vore?
- text-heavy is probably not the way to go, though a lot of flavor text (protag thoughts while vored etc) would be nice, so long as it compliments rather than supplements gameplay
- light lore through manuscripts found deep inside of "ancient beings" (worms, bosses)?
- alternate playable character with completely different playstyle (barbarian?) that keeps protagonist tucked away as a mana source :)
- alt vore is good but may want to have content settings
I know most of this is probably out-of-scope but I hope it's at least interesting