I agree Fellaini is a midfield option, and yes, Fellaini will be a good addition to bolster our midfield, but ultimately, I still think he's more of a pure CM, and we already have Cambiasso, Mariga and Muntari covering that role. Fellaini may, arguably, be better than them, but I still think what we need is a creative offensive midfielder, which can create something from midfield when needed; something like a midfield Ibra. And I believe Hamsik or Gourcuff can provide this. So it's a matter of strengthening our weakest link (the creative midfielder), then improving our already decent midfield.
Just a final note, I watched Fellaini play against Liverpool two weeks ago, and I thought he was disgusting in that match. The fouls he commited were shameful really.