I'm also desperately wanting the Black Knight to be in. I suppose I could see some attacks... and how Luna/Eclipse (whichever FE you're looking at) being his FS of course.
Though seeing how Ike is said to be as slow as he is, I can't bear to imagine how slow the Black Knight here would be. I think we ARE better off hoping that he's a boss instead. He's got the theme, haunting aura about him, sexy black armor/cape, warp magic, a health bar... he's got the whole package.
I'd love to fight the Black Knight with Ike head-on with no turn-based rules holding them back... just one big showdown. I've been waiting for another FE update for ages.