God, I love these 3 ideas! These ideas are, in my mind, exactly what the WWoHP needs in a phase 2. Maybe a bit more, and definitely a ride or two, but I really can't think of much else.
You're still off about Diagon Alley, that's why you can't think of anything xD...
I like the plan jj3352 has in mind, but London, and Diagon Alley are what I see happening... Not that I wouldn't love a Whomping Willow experience!

, but that's towards the front/side of Hogwarts, and we don't really have anywhere to go with it (other than behind Hogwarts, which wouldn't be so bad if HP had a back that could be expanded into *scratching head*)...
Here's how I see an expansion. Takeover the rest of LC. It's going anyway, we know it's coming.
Mythos = Leaky Cauldron/Diagon Alley entrance...
Alrighty, so I'm thinking everything from Mythos to Posiedon's Fury would be "London". Posiedon's Fury being some sort of Ministry attraction... I'm still debating what to do with the whole Mystic Fountain section.
Alright, so moving away from London, we go to Mytho's, and in the back is the entrance to Diagon Alley. With the crowds going through, I don't see any kind of interactive Diagon Alley entrance working properly
at first. Once crowds die down, maybe they could have some sort of event for it.
Now on to Diagon Alley itself. I see Sinbad's building being completely redone, as it's a stage attraction at the moment. Either that or bulldozing it and making it a new building for the Gringotts coaster.
Past that, I see it being similar to Hogsmeade and at the same time very different (ice cream parlour, Ollivanders duplicate [much needed], a quill shop similar to the Post office in Hogsmeade, and a few other shops that are escaping me right now...).
Oh, and possible some sort of Hogwarts Express walkthrough attraction from London section to Hogsmeade village?