I'd quite like to see Fellaini play as a pure DM once someone sits him down and gives him some actual tactical instructions rather than letting him run amok.
He's looked positionally poor and suspect in the tackle but that's partly because Moyes seemed to insist on some bizarre box-to-box role which he simply doesn't have the stamina for. If you think back to his debut against Valencia, Fellaini got plaudits for a solid display where he sat deep, broke up play and passed it on simply but effectively to the better players. It all went tits up after that, punctuated by a few injuries.
Before he was signed, there was a bit of optimism regarding his ability to play a range of roles - as a 10, a DM and a box-to-box. As above he doesn't seem to have the stamina for the latter, he was never going to play 10 for us so DM seemed the obvious choice, but I'm not convinced he played there more than a couple of times. It's unspectacular and certainly not worth 28m - it was never going to be - but that's the only role I can see him playing. He's an obvious candidate to get the chop but if that's the case we probably need another two midfielders this summer given that Carrick is out. Might be better off giving him a shot as a more restricted DM and focusing on a proper partner for Herrera instead of forcing a massive overhaul.