Might as well bump this topic as it's my own. Several people have tried to make original layouts, and some succeeded. A solid layout in a CTF level is probably better, as long as it's not too linear. A platformy level can be okay too, but don't make it like Diamondus Warzone, for example. In Battle levels there should be enough open space, so it can be a bit more platformy. Don't make too big platforms or too little ways. If people have trouble finding a tileset they can say it here too. Dunno about other levels, but it's logical that you shouldn't make Single Player levels too hard and Race levels shouldn't be too easy. Some obstacles are nice.

Well, that's my advice. I'm going to try to follow this. Some articles on J2O are useful as well.