Do you intend to have a small party of recruitable characters, or do you intend to make it a proper monster-catcher, with lots of monsters to collect? Because that’ll inform the size and makeup of your roster. It might be interesting to treat this game something like the new SMT game, where the player character is super versatile and can align himself to the game’s different elements as needed (which would be easy to mimic if your protag is a shapeshifting slime), but you also have room for 3 party members you can swap out from the game’s roster of demons. If this isn’t that kinda game, that’s fine, I just wanna get a sense of what kinda RPG this is.
Even if you don’t want a monster-collector with lots of types of monsters, you still have plenty of room to make NPCs to cover other types of monsters people want to see. If I need an idea for a monster, I usually turn to mythology/folklore for some unique ideas. Doesn’t have to be humanoid per se to have something interesting to it, either, it just needs interesting potential for character design. You could just take animals people don’t work with often and make something of them–crabs and mantises seem pretty rare, for instance.
It has been done.
Vore sounds like a useful way to justify working wg into progression. Eating humans could allow monsters to get more magical powers, but make them fat in the process. You can also work in an “evolution” mechanic if this is a monster-catcher; you could feed monsters you’ve recruited to your other monsters to make the receiving monster stronger as a sort of “fusion” mechanic.