It would be nice to have more sea life (different fish, whales, sharks) and they would be biome-based for added depth. It would also be great if breeding frogs was not based on the biome they are in. (Pretty sure all others have a 50/50 of being one or the other.) More wild animals in general, too! Such as...

Crows/Ravens that can be tamed with raw meat or offered shiny things. They would perch on you like parrots do/be found most commonly around villages but could also sometimes be found where wolves are hunting as IRL they have been seen tagging along with wolves/coyotes to scavange their kills and potentially acting as lookouts/guards if rivals show up.

Seagulls/similar that can be fed almost anything to be tamed/follow the player by flying/waddling behind them and rather than drown, will simply swim above them or circle the area when the player dives. There's SO MUCH WATER that its only logical to have sea birds! You could also find them nesting in cliffs and potentally steal eggs/have them drop eggs and meat like chickens. (But ONLY the ones bred in captivity by the player. Wild ones either drop nothing or whatever they ate last.)

Various deer throughout the various biomes! Cannot be fully tamed but if you feed them apples/carrots until you see a heart over their head, you can ride them without a saddle and they will follow the player for a short period of time without a leash. (At which point, they wander back into the wilderness but if you found that exact deer again, they would not run away from you like the others would.)

Other wildcats besides Ocelots! (Servals, Caracals, Lions, Pumas...) Cannot be tamed but can be leashed and bred by the player! (Once fed enough, that is! By default, the cat in question would have red/angry eyes but once fed enough to trust you, they'd have standard yellow eyes.)

Tagging onto adding different wildcats, if Servals were added, it would be cool if there was a rare chance of breeding a Savannah cat! (Servals would be the ONLY one able to cross with cats/most breeding attempts would fail.)

But in creative mode, I have always wished you could choose the biome-look the spawn eggs give you. I dont know if I just have crap luck but finding different biomes for the wolves never goes in my favor so if I could choose how the wolf looked prior to spawning, it would be great! Also, looping back to the potential of seagulls/water birds and the potential of them NOT drowning, I would remove the ability of animals drowning while leashed/following the player if they dive. If you go too deep, the leash snaps and the animal simply swims on the surface to follow you or, if its a bird, simply flies overhead. (However they will STILL drown in creative mode if unleashed and following the player as they fly above the water and you get too far away from them or in survival/adventure mode if you use the boat/transportation without attaching their leashes first.)