Kostas_Greece
[14] Master
It is more possible than you may think. If SCVI is actually a thing, and its release happens around the time of the FFVII remake's release, I wouldn't be surprised if Square-Enix actually gave Cloud and/or Sephiroth to appear in SCVI. They did allow Cloud in Super Smash Bros. U, they could do it again.
I wouldn't discard the possibility of characters from Koei as well. Mostly from Dead or Alive or the Warriors/Musou series. Dead or Alive is... well, still alive, and its a famous franchise, and SC already crossed the Warriors series before, as Sophitia is a playable character in Warriors Orochi 3 Ultimate. Also, Samurai Warriors goes around the same time period as SC, since Yoshimitsu seems to be involved with the daimyō Nobunaga Oda.
Wow you're right a quest character from Samurai Warriors or Warriors Orochi in Soul Calibur VI will be f****ing awesome!!! :)