When adding a player model, isn't minecraft's json used?
If so, why not implement a function with custom json models?

I don't exactly know what you mean, but no, I don't directly use model properties files, I just use an instance of the player entity. Well, the entity obviously uses the JSON's, but not in a way that would make it automatically work with other models too.

So no, I currently have no plans to add custom models, sorry.

But will keep this open for the future.

Will not add this for now, sorry.