An idea like this could work really well if the banked exp. behaved similarly to rested exp. where exp. gained at 60 would be "stored" and then added to whatever exp. you gained while leveling a non-capped class. That way you still need to play the class to use it, but the leveling experience can be eased. God knows after your 5th-6th level 60 the idea of trying a new class makes you want to perform sudoku.