I'll entertain the thought of Diagon coming to the Falls Lake area:
I'm not sure if that hotel is still happening or not, but if they're still wanting to build it, they would need at least one more significant investment to justify it. Diagon would do the trick. They should make that area what was the original vision for Diagon at USF: Have the full land with Battle at the Ministry attached to it. It would also justify having to take the Hogwarts Express down there.
Also something else to consider: during HHN, there will be a decent amount of people at the park during the day that would love to stay a little longer to do rides, but they'll be kicked out for HHN. Having a Diagon Alley would be great for the daytime people to be "contained" while HHN commences. That way the daytime folks can still stay, spend money, and have fun while not at HHN. On top of that, they can plan their day to do all of the core park attractions during normal hours and then do the Diagon attractions "after hours".
Don't know if they're serious about this or not, but I think it would be a good business move in the longterm.