The GBA lords have the disadvantage of being looked down upon in japan. Compared to the NES and SNES FE games, they're just not up to par. I already mentioned Sigurd, and Celice is overshadowed by Sigurd, so Celice wouldn't get in over him. I do feel that Sigurd would be Micaiah's main competition, but she still has a fighting chance because of the uniqueness that she would present, along with her game having not just Japan's appeal, but international appeal. It hasn't sold as well as I had originally hoped, but it hasn't been advertised (at least in America) as much as FE9, which I supposed was expected to act as a spring board. Brawl would certainly serve as a great advertisment for an international market, and before the delay I had thought of Micaiah as an almost certain addition, but as it stands the advertisement might be a little late (but still warrented).
Marth's return is about as given as Captain Falcon or Gannondorf. It's not 100%, but there are no reasonable arguements against his inclusion. By reasonable, I mean arguements that have not be refuted and are also not personal opinion.