It occurred to me that what matters isn't that a game comes out before Smash comes out (meaning FE: Radiant Dawn drops before Brawl), it matters whether or not the developers working on Brawl 1) even knew about Micaiah as they first began working on Brawl, 2) actually
wanted her in, and even then 3) they would have had to work very closely with the developers of Radiant Dawn to assure she has proper move sets, character design, et cetera.
I
really doubt this happened.
Now, I realize this is a similar argument to the whole "only games from last generation will get in" or "it's too new" argument, but it's different. This one actually explains those ideas in a way that makes sense and allows for many potential exclusions, especially when it comes to Nintendo's top 3 franchises.
Most likely all the characters in Brawl were decided in the first year or so that Brawl was in development.
Sure- Snake has the music and a few moves from MGS4, but they were working very closely with Kojima, so even though the games (MGS4 and Brawl) are coming out near each other, it was possible to implement a few newer things.
With Micaiah though, not only did she not exist before the current game she's about to be released in, but Nintendo doesn't know how audiences will take to her and if she'll be popular, and the developers of Brawl likely didn't know anything about her potential move sets as they were deciding a roster as she was likely still being fleshed out throughout the simultaneous development of Radiant Dawn.
Therefore, because the Black Knight can already be designed from what the developers knew of him from FE
oR, he's more likely.
But in all honesty, I think Marth usurps the Black Knight as well. Ike is too different to replace him, Marth is a favorite, he provides homage to the first of the FE games (and thus would make sense for the stage to be generic), and he already has everything but a final smash done and the developers aren't going to throw away that work.
So if you want to hold on to your hopes for the Black Knight, replace Micaiah with Marth.