If I had to make roster predictions:
The entire set of royals from Fire Emblem Fates is pretty much a given. So Corrin, Ryoma, Xander, Camilla, Hinoka, Leo, Takumi, Elise, and Sakuya. Azura is highly likely as well. This a pretty balanced and diverse set of characters, many of whom have unique weapons or rare classes, so they would add a lot to the gameplay.
Shadow Dragon has a pretty specific list of major characters who are particularly prominant in artwork and such: Marth, Caeda, Ogma, Navarre, Merric, Linde, Minerva, Tiki, Camus, Nyna, and Gharnef. That is the list of characters for whom Nintendo commissioned promotional artwork for Shadow Dragon from Masamune Shirow. I would add Hardin and the White Wings in as also being likely additions (though I can see the White Wings just being called in as assist attacks by Minerva or something).
Awakening is a bit harder to guess at. Chrom, Lissa, Robin, and Lucina are all but confirmed and super obvious picks. I would say that Frederick is extremely likely as well, if for no other reason than to have an axe-using cavalry character. Virion is a likely pick, and so is Tharja due to her unfathomable and inexplicable popularity.
And that comes to 30 characters, which is probably on the high side, but not outside the realm of reason.