Me personally, I can't really think of any Nintendo characters that I'd want playable. I guess I'd want Chrom or Robin to be playable. (Moreso with Chrom being playable and Robin being an assist trophy or something similar.) If they brought Roy back, I'd prefer for him to be redesigned. Maybe they could use his Awakening look and give him a new moveset.
Hrm...Yeah.
On a side note, for bosses, I'd love to see Medeus or Grima. I just want to be able to kill a giant menacing dragon.