With the mention of PvP eventually being a
possibility I think that it would be neat to have the character fixes and levels be separate from the campaign. In this case, per winning a match, your character class that you had used, would very slowly increase in power. Say, about every 20-30 wins, you get a bit of a damage bonus.
This would especially be neat if there was a form of group PvP, having a priest and ranger in the back with a paladin tanking, etc. Allowing for more diverse and strategic PvP sequences, as people could form sorts of guilds or groups to PvP in.
I hardcore play a lot of MMO's, so it may sound a bit like that, but no worries if it's not a loved idea, comments and criticism are always loved.