With the announcement of the Cave & Cliffs update, it would be cool to get some new, cave-specific mobs. I propose that we get a new variety of bats spawning in large caves deep down (not the deep dark). Contrary to their smaller relatives, large bats do not tolerate your presence - they will attack you by swooping down in a way similar to phantoms. Large bats will spawn in groups of 3-5 in large cave rooms and will hang under the ceiling, sleeping until they are disturbed. As they are creatures of the dark, they steer away from bright light. Standing close to lanterns, torches or other light sources can save the player from being attacked (in the same way hoglins steer away from warped fungi).
Visually, large bats would resemble normal bats with smaller eyes, larger ears and darker fur. They would be slightly smaller than the player (ca. 1.5 blocks in length)
The addition of large bats would add new, cave-oriented life to the newly reworked caves and make use of the large open underground areas in a way none of the existing mobs do.