The exp bonus is interesting. You'd have to slow down the leveling process overall, around ffxi speed I'm guessing, but if you level some jobs more evenly than you speed your process up considerably. I wonder how slow it would need to be up front, hopefully not so slow that it discourages new players. Would definitely need a cap to how much bonus you could gain so that you're not basically power leveling yourself.
If you want to make lvl 20 dungeon gear more relevant, than that equipment is going to need some serious longevity, and a leveling process where you can't get from lvl 20-25 in a couple hours. I don't think I like the idea of making it take longer to get to endgame (which most people seem to want to do) just to give more sense of relevance to a dungeon or quest that is by default made to be outgrown.