Joshua from Sacred Stones would be a neat addition, seeing as you first encounter him as an enemy. Plus we could always use more Magvel units.
We have a ton of Awakening units so far, but Aversa would be a really neat concept, as far as I can remember we don't have a flying tome user yet. Probably the only one I actually expect.
This one is the furthest reach, but getting Idunn as a beast unit with a breath weapon would be interesting and be a neat obscure character.
If IS wants to continue pushing Shadows of Valentia, we could maybe get either Dean or Sonia. Maybe both but you only get one? :P
Fargus. He never got to come with us, I'd love to recruit him.
Finally, Lyon would be a stellar addition, but probably as likely as getting Idunn, so next to none. ^^;