Honestly, I think a Harry Potter MMORPG would be pretty sweet. It should emphasize problem solving and teamwork over "kill X amount of monster Y and bring Z drop". You pick your House and gain experience in working with different areas of magic.
Or, I suppose this could do just as well outside the HP universe. The important bits are, you should advance solely or most quickly by beating up monsters (if that's the case, why are you going to school?), and the school should be huge and you should have free reign to explore it and the surrounding area. (Although maybe it shouldn't be quite a wide open sandbox, per say. Some portions of the school and the world should be unlocked gradually throughout the game(s).)
I recommend that you think on this idea more seriously after you read Harry Potter and the Methods of Rationality.
Because holyshit it does awesome things with the HP universe, pokes fun at the books, and takes the plot where it wants to go. You'd want to design a game where those possibilities exist, as well as even more.
Not just where those possibilities exist, but where
Greater Harry has already finished all his 7 epic years of Hogwarts
without dieing or being exiled and has sent the entire government and magical world into
progressive chaos! Suddenly, your magical schooling is
no longer a goal in itself but instead one
epic tutorial you barely notice is even a tutorial that prepares you for fighting the
deathless war in service of the glorious
Light Lord Scientist Potter-Evans-Verres! In which your job is to use your many powers of technology and magic to break down the masquerade, overthrow the corrupt, and bring about a new age of understanding and wonder.
Which, of course, primarily values making good choices a la King of Dragon Pass and Planescape:Torment, but with other interesting game systems either cloaking it (such as a first-person shooter mode for missions, schools games, or the School Armies) or acting as resources for it (like the ad-lib magic systems in those forum games
lately and
long ago, where experimentation is key*). It would make a hard game...a thoughtful game...a game that makes you greater than you were before; much like the work it would be based on.
(Not the work the work is based on; that's too much of a children's book).*I recall published games exist like this, but I can't seem to remember any of them...