Hmm, defending your title after becoming champion would be an awesome idea. Nut then again, it would we rather awkward if some random NPC beat you and then they became champion, and now you need to beat the Elite 4 again to just challenge this guy so you can become champion again.
Obviously this is suckish dialogue, but I'm sure you guys know what I'm trying to say.
I really like this idea. This would be perfect if they implemented this in future Pokemon games.