Post-release DLC packs a la Hyrule Warriors are pretty much the way to go. I could easily see them doing Binding Blade, Blazing Blade, Sacred Stones, Path of Radiance/Radiant Dawn, and Shadows of Valentia packs. At the absolute least I can see Roy, Ike, Alm, and Celica getting in as DLC due to their amiibo.
How I see it is this:
- If you want a major character from Shadow Dragon, Awakening, and/or Fates, they're almost guaranteed for the base game.
- If you want a non-major character from the three mentioned above, they have a shot at the base game due to them being the main game content source.
- If you want a major character from Fire Emblem 6, 7, 8, 9, 10, or 15, they stand a good shot for post-release DLC.
- If you want a non-major character from the six mentioned above, you're likely out of luck, since I'm not sure the DLC packs will be that extensive.
- If you want any Jugdral character at all you're probably SOL, sadly.