THe main question for the 'in-betweener' is if it's to be a very long single aisle, like the B753, or if they opt for a small widebody, like the B762. Pros and cons on both sides, but if they can crack that, the rest should hopefully fall into place
Personally, i'd say the A322 is the ideal way to go, from a pure cost level, but then again, a thinner A330 for pax comfort/convenience (having 2 sets of overhead bins)