So what FE antagonists have a chance in making it to the roster? I've been curious and I barely know anything about FE. What about Black Knight and Ashera? Those were villains in the games Ike were in right?
Ashera was the final boss of Radiant Dawn (spoiler) and Black Knight is a recurring, extremely powerful enemy, who eventually joins the player's side as a part of Micaiah's faction (spoiler). Given just how brutally strong these two characters are canonically, I don't feel like they would work. Also, Black Knight is naturally a large character model who in-game is essentially a tank that is hard to damage and just deals heavy hits. I think Black Knight is so slow that he wouldn't be enjoyable (general class characters don't really jump), and really compared to the protagonists like Ike, Chrom, etc. I feel like the Lords and other protagonists would have priority over him in being added to the roster.
Also, Ashera being the final boss would be somewhat difficult in the sense that she:
a) is extremely powerful (although extremely powerful forces like Mewtwo and Ganondorf have been put in, so this may be irrelevant)
b) As a final boss, she doesn't really move that much at all, instead, she has an army of elemental "spirits" which surround her, also, she only uses one attack that deals major damage, and while certainly a moveset could be created for her (Like what Sakurai did with Captain Falcon), the fact that she doesn't move in FE poses a problem from the standpoint of balancing her mobility.
c) Doesn't really have much of a following/fanbase, which is a problem faced by many FE final bosses (aside from the fact that many of them have such unusual body types that they probably wouldn't work in Smash regardless) because typically each game in the franchise has a different final boss that is incredibly powerful. It doesn't help that she essentially splits this responsibility in the Tellius Region (as Path of Radiance and Radiant Dawn took place on the same continent) with King Ashnard of Daein, who was the final boss of Path of Radiance (the first Tellius game).