Currently, all shopkeeper mobs (except shulkers in certain states) are affected by gravity, both when being spawned (to place them precisely on the block below) and continually whenever there is no block below them anymore.

Idea: Allow certain mobs to hover in the air, i.e. not be affected by gravity.
This could either be hardcoded, or be configurable. The latter is currently preferred, since this gives admins additional flexibility to configure this feature however they like. However, on Minecraft updates, new flying mobs are not automatically added to the config file then.

By default, this could include the vanilla flying mobs found here: https://minecraft.fandom.com/wiki/Category:Flying_mobs