Why not be able to merge classes. Say you become proficient in Necromancer/Mesmer. You have to have the secondary. You could then go to a certain outpost and have your character fused. It would take some qualities form each profession you had before the merge and gave you skills. I can't think of a good way to combo the N/Me in such a way, maybe someone can be creative for me.
But when you fuse you gain abilities form both but end with one class that is slightly better then both classes as they were. You could stay as this class until you do soemthign else as it to split back. Even if it would make more sense for it to be a permanent change. This would give players a difficult choice but yet open more possibilities gameplay wise.
k
