What I've always envisioned for an expansion is the following:
Phase 1: Forbidden Journey gets a brand new scene (perhaps in the last dome where you would fly over Hogsmeade instead of the lake) and Dragon Challenge gets a new paint job as well as a complete refurbishment of the queue to make it look like the stadium in Goblet of Fire (you wouldn't be boarding a real dragon--you're actually going to be helping the crew catch the two dragons gone loose with vehicles that look like its counterpart, therefore guiding them back to the stadium) with fire effects and whatnot, and the new rockwork facade to block the FJ building as well as to make use of the hideous green wall--the greenhouse section is completely closed off visually from inside and out, and the remaining section of FJ ride building is rethemed to Jurassic Park.
Phase 2: The North entrance to the Wizarding World (not Hogsmeade's arch anymore) is installed as a pair of looming rocks with a fiber optic effect declaring WWoHP's logo (a large rock on the other entrance is placed that has the same effect), the Hogwarts Express plaza is moved to where the magic fountain is, and behind it is a life-like simulator on the Hogwarts Express wherein you partake in a guest viewing of the surrounding area of Hogwarts and Hogsmeade complete with free candy (each ride is half an hour and so the experience is separately ticketed--there are two simulators within the ride building), and the place where the Express plaza was is replaced with Madame Puddifoot's and a new locker area for Dragon Challenge.