The new SC should include all the modes and that were in SC2 and/or SC3. Also, it would be awesome if there was a story mode just like the one in SC3. In addition to that, another Chronicles of the Sword or Weapon Master mode (either of them) would be great...
As for the characters, there should be no guests, but the old abandoned ones should make a return (Li-Long, Edgemaster, Necrid, Nightterror, Inferno etc). Additionally, all of the canon characters must come back and most importantly, KEEP SIEGFRIED, PLEASE, PLEASE,PLEASE!!!
About the music now... The new soundtracks should be more like the ones in SC2/SC3. If some SC2/SC3 soundtracks would make a return, they will be welcomed (Hellfire, Forsaken Sanctuary, etc).
Concerning Siegfried now (assuming he returns), please don't kill him again... Make him suffer a little bit... I mean that, he should somehow find out that Soul Calibur is as dangerous as Soul Edge (Taki is right)... Ohh... And please, in the end (not in this SC nessecerily) let him find out that the only sin he commited was killing accidentally his father, the rest of the sins were just Soul Edge's against his will... Also a great ending for him would be, the freeing and resurrection of all of the souls inside Soul Edge and maybe the magical resurrection of his father... Last but not least, DO NOT MARRY HIM WITH ANYONE, NEVER. He is better alone... So, please forget about SCBD's Sarya (or whatever her name)... PLZ, PLZ, PLZ, PLZ, PLZ.
As for the moves and stuff, I'm not an expert so, I'll leave that to the experts here who know what's going on. All I can request about that is balance.
Well, silly or not, that's what I'd like to see in the new SC. Thank you for this GREAT opportunity!!!
PS: The series should last a very long time with loads of sequels... Besides, THE LEGEND WILL NEVER DIE!!!