Three Broomsticks is just making more money because Potter is the hottest thing on Orlando right now. The food is just a more or less version of the restaurant that was there before it, with the addition of chicken, pot pie, and shepards pie, and of course wizard drinks... It is nothing to scream about. As for Mythos, it is still popular and UO is smart enough than to just get rid of all it's success for Mr. Harry.
I would love an expansion. But I am being realistic, in thinking of theme park terms, and not being quick to think that UO just wants more and more money... I don't see Potter expanding over Lost Continent. They'd have to close everything down, get rid of their most popular restaurant, and there is not even a very reasonable and appropriate design or concept to expand Potter with what it is now. They can't open more stores... what else is there? All the merchandise is there, and they are getting more and more of it... in which they do have room to put it. In the shops, in the carts, in the stores at the front. They already have a very popular dark ride attraction, and two roller coasters, one being an extreme thriller. They did claim that they promised to add in Deathly Hallow elements, which IMO may include a few minor additions and updates here and there. But nothing as big as to take over one of their lands. They need to clean up LC (which clearly they have been doing... looks to Mythos structure) and add an upscale attraction.