There are limits to OC's, one of them being strength, if an OC is way too strong, it might make the story/fandom fall apart, and you don't want that, it can also make the story/fandom, boring, there's nothing else that you could really do with them

Another one is color pallet, the wrong color combinations could hurt someone's eyes, too many colors would be confusing, you can use only 2-3 colors which is okay just don't use the wrong colors

Theres complexity, if you added too many things to a costume, it could make it hard to draw them, if there was barely any detail at all, that's fine, because less is more, and over-detailed OC's wouldn't last very long

This might be the last one but it's the back story itself, bad backstories would make the story fall apart and good ones might make them last a while (I don't really know how to explain this)

I'm not done yet