My initial thought was that Logia Vs. Logia fights could be extremely boring and ridiculous when both combatants will just fling their respective elements at each other. It will be complicated, but it can be done. I don't think people can exploit such a system by making chars who specifically target other chars simply because you can refuse to fight anyone you don't want to, it's a free RP, and since most if not all Logias will have weaknesses, no Logia-user would be unstoppable.