The Engine

[Runouw]

Since the game's online mode runs on P2P, it wouldn't take long at all to transfer an entire character through the internet. (A completed character is likely to be less than a megabyte)
Perhaps in online mode, there should be an option where you can allow or disable custom characters, if you disable, it won't let people use custom characters against you in a fight.

And since online mode wont be a blind "Searching for opponents" like it was in brawl (it will use a "create room" and "join room" system). I suggest the the creator of the room can choose if custom characters are allowed, and people who join can only play as the custom characters he has on his game. [Pretend joining a room is like going to someones house and playing the game off their computer as another player. That means you can only use the characters he has on his computer. If you don't want spammy super-powerful custom characters to ruin online mode, create your own room where the characters you have are fair.]

And I guess if you beat Classic, Adventure, and All Star with every character, we should allow people to play those modes with custom characters. There's no loss in allowing that is there? We could even have it allow you to play with custom characters from the begging, but just make sure that you don't unlock or gain anything from it (that means no coins or cards gained from playing it)