Mirallas is good enough to play for a top side, certainly, but pretty much every top side already has at least one or two players of at least equal ability to Kev. As has been said, there's Cazorla and Sanchez at Arsenal; Di Maria and Mata at Utd; Hazard/Oscar/Willian/Schurrle at Chelsea; and Silva/Navas/Nasri at City.
Abroad? We can forget Real and Barca in Spain or Munich in Germany. Athletico have Turan and Griezmann. Valencia might be more realistic but they have the likes of Rodrigo and Feghouli. Most other Spanish sides don't really have a more realistic chance of CL than us, although I can see Moyes making a move if Sociadad don't punt him. Then there's France, where he's already flopped. He wouldn't get a game for PSG and I doubt there's much money in a move anywhere else. Then there's Italy of course, where football goes to die.
He'd be a bit part player for most teams that are a level up from us, meaning he wouldn't get a run of games and wouldn't get into any sort of form. He couldn't wish for a better position right now: a key player for a good team who - let's pretend this season is just a nightmare - aren't far off being a CL quality side.