I'm thinking that we're just going to have Marth and Ike representing the Fire Emblem franchise in Brawl. I know Fire Emblem is popular enough to warrant more characters, but due to the nature of the series character choices just really start breaking down after Ike and Marth.
Sigurd seems like he just doesn't bring enough to the table to be anything more than a costume (unless he gets to ride in Brawl on his horse, of course), and either Micaiah or Roy would be cool due to light magic and fire, respectively, although both of them are slim chances for their own reasons. And Black Knight, well... his only real merit is that he managed to get enough of a fanbase to even be requested, which is impressive for a relatively minor character like him, but not enough to warrant a place in the Brawl roster.