For all of the various merger rumors being chucked around on these forums, there are only two obvious ones out there so far as I can tell. None of the legacies will be allowed to acquire anyone, and even SWA would have an uphill battle to get approval for an acquisition (looks like they are going back to organic growth with their expanding international services anyways).

However, a good case could be made for JetBlue and Virgin to merge due to their fleet similarities (E190s aside), similar cultures and customer segments. Also, Spirit and Frontier would also be good merger candidates for the exact same reasons.

Although I'd be happy to see UAL buy new cut rate C-series I think mgmt has already tipped its hand by snapping up used narrowbodies. There should be plenty more of those coming available as carriers from weaker economies start shedding aircraft to survive (e.g. Aeroflot is trying to sell roughly 40 aircraft). We are unlikely to get new/used E190s either, at least it seems that way if you've watched Brian Z's comments on fleet planning in the UpFront News.