I don't know if it's possible but I'd like to be able to make custom models that you can set per mob type
I.e. Make a custom model for a creeper and all creepers will be replaced by that model

The way it'd work on servers is by only allowing server operators to change mob models

[Assuming the issue is still relevant]
You may want to look into the Entity Model Features and Entity Texture Features mods, through which you can replace entity models via resourcepack.