Well, if you look at the whole game, there is one irremovable base line to exp growth; which is killing the open world monsters. Everything else added the game is somewhat of a supplement to this and will most likely be used to accelerate leveling beyond this base line. Basically, sidequests will be better than killing world monsters if they are intended to be a source of experience and implemented correctly. Otherwise the situation will bear no change.

OP raises a good point about sidequests and leveling multiple classes though. Obviously a lot of quests would have to be added should this become a significant source of experience. And you aren't guarenteed an even distribution of quests either. I'm willing to wait and see though.