Model files exist for all items and they can be edited freely. Some aspects of block models, on the other hand, cannot be modified.
The models should be fully modifiable when in block form as well.

Affected blocks:

• (wall) skulls/heads
• chests, trapped chests and ender chests
• (wall) banners
• (wall) (hanging) signs
• shulker boxes
• beds
• enchanting table (the book cannot be remodeled, the table itself can)
• lectern (the book cannot be remodeled, the lectern itself can)
• bells (the bell itself cannot be remodeled, the supporting attachment can)
• end portal (exists in block form only)
• end gateway (exists in block form only)
• (flowing) water (exists in block/fluid form only)
• (flowing) lava (exists in block/fluid form only)