I did some measurements on google earth and I used the Three Broom Sticks as a guide for the size of a restaurant. You're right it would have to go behind FJ. There is no room by Hippogriff. I do wonder how they would have you enter the restaurant though. You would have to walk through the greenhouses and to the back of them to get to it. Then would they build a facade for and how would that look there? Also, I think it should be a table service, maybe a buffet or something, then maybe we would finally getting a full time sorting hat. They could've used some of the dragon challenge land to move the FJ building by Hippogriff but it appears it's too late now.