I disagree. I think Sakurai said he wanted to add more rival/antagonist characters to SSBB (Samurai Panda's translation thread, I think it was). Black Knight is the best candidate for a Fire Emblem rival imo. He's much more of a rival/antagonist than MetaKnight, that's for sure. Yeah, he isn't the primary villain/final boss, but I don't think there are really too many final bosses in Fire Emblem that are reasonable, and I really don't think they will add an antagonist from any Fire Emblem in the game unless there is the primary character from said game also in (example: Nergal was announced and Eliwood, Lyn, Hector, or anyone from there army isn't, that would be very strange, not that Nergal has a chance at all. How about them announcing Black Shadow with no Captain Falcon? Or SquareEnix making a Dissidia: Final Fantasy with Sephiroth but no Cloud?).
I recall Sakurai saying something like that as well, however that does not mean that every single franchise has to have a villain/antagonist like character now does it? All of the villain/antagonists have been of central importance to their respective games, the Black Knight is not. Why should he get in over more important characters to the Fire Emblem franchise?
How would you say the Black Knight is much more of a rival/antagonist than Metaknight, I'm curious about your reasoning to get to that conclusion.
I'm pretty confident that, with the exception of retro characters (Ice Climbers) and characters from series that don't have a reasonable antagonist at all (Captain Olimar, Animal Crossing character), I think every character from every major Nintendo series will have a 'rival' character to match.
Yes, I am a strong supporter of Ridley (or even Dark Samus), King K. Rool, Black Shadow, and Wolf.