Cause having 77 out of the 174 total characters be from 2 out of the 11 games (remakes counted with the originals as they have the same characters) makes total sense when other series still haven't even had their main heroes added.
Knowing the dataminig when the app came out, Daraen and Tharja will be there. On today's image appears Lissa, so the 4th one should be from Awakening as well. Maybe Chrom or a brand new hero like Sumia or Owain.
Pokemon GO
Fire Emblem Heroes
Fate Grand Order
Dragonball Legends
Dragalia Lost
IV Calc
Heroes