So just leveling your main class to 80 using your normal means of exp would get your secondary to 40 and third class to 20.
Technically, the increase in XP required for levels is more exponential than linear so by the time you have your first class at 80, your second would be at 69 and your third at 62.
This strikes me as somewhat overpowered and would likely not ever appear ingame.
Ok, you got me there. Hadn't thought about the scaling of experience through the levels.
Either way you get the idea, the point should be that by leveling your primary class you should also be gaining experience on atleast your secondary class in some way, shape, or form. You really shouldn't have to sacrifice the experience you gain on your primary class to level up your secondary class.
Another concept I had been throwing around would be something to the effect of this:
Zone Quest Reset - Usable only once per month - Allows all Non-Epic quests to become available to be completed again for a selected zone. If quests had been completed or were in progress at the time of this item being used, they will become available again to the player. (Costs probably about $10 USD)
So this item would allow you to choose a specific zone once a month and reset the non-story quests. This would allow you to go back through a zone (Like TB) with your secondary or third class for some level appropriate experience. This would also encourage players to actually play their secondary class to get through the zone or buy tons of Home-Sweet-Homes.
But again, just a thought, and probably will be nothing more than that.