I think Moore and London would only return if WWE decided to have a Cruiserweight Intiative and to bring the Division back.
It was only after TNA ratings for the Knockouts were scoring higher than all the other TNA segments that WWE decided to bring in a second Women's Belt. So logically if TNA were to properly reinvent the X-Division and score well for those segments, in an effort to compete/better them I could see them trying to restart the division, and obviously then they would re-sign former cruiserweight guys.
I can't see Palumbo making it back again, they have given him numerous chances (admittedly they have screwed some of them up), I just can't see what they would do differently with him.
Umaga was a case of failing a drugs test and refusing rehab (I think), so once he gets himself sorted I definately see him back at some point, he is a great wrestler for his size.