The basic premise of the champion is that as he becomes corrupted as he gets stronger (levels, kills, assists). As the champ reaches certain thresh holds of corruption (level 6, 11 etc or a certain amount of kills) the character model changes and new or altered skills become available. The available skills before becoming corrupted would no longer be usable.
I think it would be kind of cool to start off with an honorable, good champion and see him become more and more evil and corrupted as he gains power. The idea is that by the end of the game you're playing a different champion. I think there are a lot of different ways this could go.
EDIT 2: Wow, there are lots of great ideas here. I believe there are a lot of different ways this champion could work. Someone mentioned a morality scale based off kills vs. assists and deaths. An interesting mechanic. Others have mentioned an evolution system which would also be cool. Another idea going off the concept of becoming more corrupt as the champion gains power is having a built in Mejia's (as the passive possibly) where the champ gains corruption and its benefits as kills and assists are accumulated and then cut down for deaths. Keep the ideas coming.
EDIT 3: Apparently this is a pretty good idea. Riot pls... Hire me...