Now, this is likely already been suggested, i've actually know where the hooks can be placed for this. not being a forge developer, it's best to just put it here for devs to read whenever.
1: Custom render passes.
Within RenderLiving; there's an area where it calls multiple render passes, right now it's set to 4, adding a getRenderPassCount() integer there would give modders the ability to add their own level of detail without adding their own classes.
2: Custom Armour render
By using the first method, there could be a system for armour rendering. Each piece would have a getRenderPasses(), and the getRenderPassCount() will add all these together, on rendering them, it will pass to the Item itself and call the method(by counting it's own render passes from 0), an interface containing a pass count and render script would mean each item would have it's own level of layers, colours, etc.
3: Weapon reach support
As this is suggested to weapon modders frequenty; it's not actually supported. and means of extending reach is real buggy and has issues. while decreasing is impossible. within (not sure anymore where it was) there was a script to get the players range (in creative, it's larger if not, about 4), adding a getReach() in item can be passed through this to allow flawless weapon reach support
*just chucking this out here, you may have already considered these methods.