I think Mcreator should add functionality for changing things that are already in Minecraft like adding physics

You’d probably have to mixin stuff like that

https://github.com/SpongePowered/Mixin/wiki

Yeah MCreator seems to be mostly made for adding new stuff rather than changing already existing mechanincs/blocks/items.

You can change existing mechanincs/blocks/items using global procedure triggers, item extension mod elements, recipe overrides, and tags