EmceeEspio
Not the Break Man
Not that anyone I know plans to play Federation Force.Probably due to space issues. Game has a lot of voice acting and is really huge.
Hopefully, they can release a DLC pack with Japanese voices, unless they ran into licensing issues regarding the usage of the voice clips outside of Japan. Given they managed to bring the option for previous games and released Japanese trailers outside of Japan, that doesn't seem to be an issue for Nintendo.
Curiously or not, those iconic characters have major NPC roles in Smash 3DS/Wii U.
Metroids, Mother Brain and Dark Samus are Assist Trophies and Ridley is a boss. Metroids also appear as enemies in Smash Run (along with a few other recurring Metroid series enemies such as Geemers).
Of those, Dark Samus has the best chance of being playable in future Smash titles. I agree that Sylux may become important given statements regarding Federation Force and Prime 3's ending.
Sylux seems cool but I would always prefer Ridley or Dark Samus for Smash.
Or you know, all three.