Now, I realise this has all been said before by many a hopeful fan, forum-newbie and veteran alike, and is looked at as nothing but foolish dreaming, but I think it's a good enough idea and just possible enough to be brought up officially in a thread.
Remaking Morrowind with Oblivion. Now, I realise it would be a massive effort, but most all of the material has already been supplied to you. It would be like Morrowind, but with Oblivion perks. There may be a few translation problems, but that can be overcome easily if you just think of an equivalent or a proper substitute.
The only concern I have is if this is will have copyright issues, as it most assuredly will. But if done tastefully and with the proper permissions I beleieve it won't have this problem.