- Biome-specific block generation.
- Ability to make End-exclusive blocks generate. (Also option to make them not appear on the main End island, for people who want blocks that require visiting the End City to obtain.)
- Custom enchants. (Would require a lot of scripting or settings in the GUI. Or events. Probably all three. This would be very advanced.)
- Events when a custom recipe or smelt recipe is completed.
- A block being able to have more than one custom drop.
- Configurable drop rates of said custom drops.
- Ability to support items from other mods, even if they're not MCreator mods. (EG Adding simpleores:onyx to be able to use the Onyx item from the SimpleOres mod.
- Ability to put a generic OreDictionary item in a recipe. (Example: Using ingotCopper in a recipe would allow the use of copper from other mods.)
^ If recipes contain items from mods that aren't present, the recipe would be disabled.
- Ability to put items in multiple Creative tabs.
- Ability to sort where items are in Creative menus.
- Ability for tools to have specific abilities. (EG a tool that always smelts items when mining with one)
- Ability to edit existing vanilla behaviour. (EG Being able to edit Fire Aspect to auto-smelt when mining, or making End Stone Bricks have a different hardness value.
- If not already possible, a way to make blocks turn into other blocks when coming into contact with water.
That's all from me for now. I'll probably come back with more ideas some time, if MCreator's devs listen to these ideas.