i think the idea for a Phase III isn't overkill, unless they really did make a new park for it. i think that's overdoing it, but making additions and minor expansions to both areas is just fine. i think i literally need a Ministry of Magic (at least a walkthrough) in London, but like it involves you doing a walkthrough and then suddenly the Ministry is under siege by the Deatheaters and you have get out as fast as you can. it could borrow the final scenes from Order of the Phoenix when the kids come to the Ministry but for some other reason (idk i'm rambling the idea's been in my brain for a long time lol).
also, a Whomping Willow ride would be cool too! idk what you could do for the Shrieking Shack though. BUT A FORBIDDEN FOREST DARK RIDE WOULD KICK SOME SERIOUS A's!