I think you should add amphibians to the game, they would spawn in ponds and rivers and would be a docile mob, and if you add axolotls they could be a key in making a regeneration potion or something like that as they naturally regenerate. You wouldn’t have to tame them but I think they could just be a cool thing to see. They could also spawn in swamps maybe.
Frogs for jump boost + slowfalling or something else would be nice (similar to Turtle Master potion). Or Frog armour (and extra bouncing off of lilypads). And a bunch of other creatures would be cool (meat, potions, armour and more). But I never know if we really need them or would come across repeats of features we already have or just a higher level of one in the case of say jump boost for frogs being jump boost 3 or 5 for example compare to how much rabbits allow for survival but commands obviously can be controllable for it.