Fight Khan in Manchester for probably the best pay day. There's really no other fight to entertain. The Marquez fight isn't really that interesting to me. I think Marquez beats the **** out of Hatton - especially if he's overly aggressive.
Retire. I'm a Yank who enjoyed watching Hatton, and I would hate for him to come in completely weight-drained or overweight and just get picked apart. I'm not sure he can beat anyone significant at this point.
Hatton's weight issues and his fighting style means he's clearly not at his peak anymore. If he does fight on he probably doesn't have more than 1-2 fights left depending on who and how he fights. He should stay away from any of the young guys like Bradley or Alexander. A fight with JMM would be big but Hatton loses that one. A fight with Khan would be big, especially in England, but at this point Khan would beat Hatton. A fight with Barerra would be winnable for Hatton but I don't know if Barerra even plans on fighting again. A fight that would make a lot of sense for both guys would be Morales. It could possibly sell well in England and both guys have seen better days and it might actually be entertaining.
As much as I enjoyed watching Hatton a few years ago, and I love the hype and the attitude he brings to his fights, at this point I'm inclined to say "retire" as well. You just can't balloon in weight and go up and down like that for years and stay in good form. Like someone else already said, at this point he'd get picked apart (and probably even knocked out) by most of the top-level 140-pounders.
He should retire but I wouldn't be against him having one final 'farewell' fight in Manchester, maybe against Malignaggi so he can look good again without getting his chin tested.