I know this idea somewhat overlaps modding, but it is potentially a different category. When I was a kid playing BK, BT, and DK64, I wondered, wouldn't it be cool if I could make my own levels, in game? What I suggest is a simple in game level editor, specific to the game and its mechanics. That way someone wouldn't have to learn something like Unity to mod a world into the game. A good example of what I mean is with Portal 2's puzzle creator, where people can learn it and it is fairly simple and specific to the game, and people can share their worlds on Steam workshop. Before, you had to use the Source engine and, like I said, mod your worlds directly into the game. With a simplified and integrated level editor, customization and creativity would be open to a larger audience, including kids. People could potentially make their own assets and import it into the editor too.