User blog comment:Crimson Storm Mich/Future Units Preview v2/@comment-27362456-20160816165101/@comment-27362456-20160817040450

Actually, not a lot of people wanna see the old characters. As for the selection process I think they try to follow the votes, but if there's units that are related in some way then they'll release them with together.

e.g. Elza with sister Alice, Kira with fellow disciple Feeva, Quaid with fellow GoM Ivris and Shida whose final battle was with Chrome.