wouldn't it be insane to manage the difficulty?
I mean depending of the various add you offert, quest can be easy as cake or nearly impossible. For exemple if you purchase let's say a new spell pack, and a new continent quest, what about the mob pack? depending you have one or another ( spell or mob) calibrating the difficulty can be hard. Another point is the world, you need to consider it as a whole. Putting stuff in it and it interaction with other adds need to be calculated carefully.