If FFL ends up being the presumed MoM- the only way I see them adding more HP is if they don't actually use all of the DC plot. If we get Forbidden Forest and the Shack and some extra shops/snacks- and they only use 5 of the ~8 acres- then I think we see a final HP expansion as that area will be marked as an expansion plot. And quite literally, what else could they put back there in the middle of Hogsmeade other than potter? That's when we see Great Hall and maybe another attraction.
But I'm greedy- Im like Veruca Salt. I want it all and I want it now. And a rebuild/move of Hippogriff while we're at it.
I disagree on the point that they shouldn't use FFL for more potter. MoM is an intricate part of HP. By not using it- we'll always be wondering what it would have been like.
Likewise, Forbidden Forest, Shrieking Shack, Black Lake and Great hall are all intricate parts of HP. I'm pleading with every HP bone in my body that they incorporate all 4 of those with the DC expansion.
And I'm not even this massive HP fan- just a theme park one.
Think about it like this- we always get "snippets" of areas or movies or land. This would be the first that we get everything. From 7 books and 8 films! It would be unprecedented. For this reason alone, I hope they do it. Then "Potter expansion" would never be mentioned again because there would be nothing else to do. With that and Nintendo anchoring USF- they can get onto fixing everything else that needs it.
EDIT:
I'd be fine with that- and if they use a little land for a table service British Fare as part of London- all the better.