Blank block/item (was: Easy custom textures)

Recently, there has been a lot of hype about using resource packs to change the textures of items and blocks, but what if you could do it without resource packs at all? If there were two items in the game with the same mechanics as player heads, but with one model based on an item, and the other one based on a block, it would allow server owners and command block creators to make custom items and blocks without asking that people install custom texture packs because the items would already have a texture!

Another thing you could do with these blank items is give them custom properties like, for blocks: Immovable (as in obsidian), Unbreakable (as in bedrock), Transparent (as in glowstone), and Conductive (as in redstone wire).

For items, you could add properties like: Do not show enchant (so the enchanted effect would not appear on the texture), Cannot move (can't be moved in inventory space in survival), and maybe some other stuff I haven't thought of.

Something great would be if you can make the items larger than inventory slots so you could create new UIs using these items.

Just some ideas.