The problem I can see with this is that new players could just powerlevel themselves by creating 30+ characters right out of the box. I think that if something like this were to work it would have to be based on something like number of levels gained or number of jobs at max level, per character, but on an account wide basis. And any increases to the exp bonus would have to be a gradual enough process that players wouldn't see an account upgrade as a possible p2p boost, but at the same time still give a noticable benefit to players that like to play a lot of alts.