Bats are cool. They make the caves seem bigger and spook you. The tropical fish make the ocean look alive and bottomless. There needs to be a bird and rodent class of mobs, much like the bat who drops no loot and makes noise periodically. But maybe, apply the mechanics of tropical fish variation to birds, and make them biome specific. And birds and rodents use the silverfish method for spawning and resting, able to merge multiple birds in one leaf block etc.. Their population would be based on tree amount per chunk. Being random-behavior ambient mobs, I would not let them trip redstone. I think this would give a larger sense of ambience overall to the game, And really tie in to the look the game is moving towards, in my opinion. Hope you consider.