I vote Potter as well. I love the HP series and have read them all aloud to my kid twice so far. I can't wait to take her. I'll probably feel the same way about a Star Wars expansion, whenever and however that ends up happening, and for the same reason: It's something I can share with my kid because it's something we both love.
I honestly don't think Disney has anything outside of Star Wars that has a chance of competing with Potter World in terms of drawing people. Tweaking Tomorrowland is something I'd love to see, but it's peanuts to adding an entire "land" based on a beloved franchise. I know, I know. Some people are sick of franchises, and that's fine. I'm just saying that Potter World will garner huge amounts of publicity and, as others mentioned above, will draw people away from the Disney parks. I just don't see how Disney has anything outside of Star Wars to draw that kind of attention. I honestly hope they don't do anything severe to Tomorrowland (I'd rather see Star Wars get its own land somewhere, if possible), but I imagine they'll need to do something big as a counter.