They already did this in SoulCalibur IV, the companions who would be with you and/or fought against in story mode. The hope can exist that they would be unlockable, at the very least in template form, but perhaps as a Bonus Characters slot if, like in SoulCalibur IV, despite being creations, some of them can't be recreated faithfully, due to them bending the rules to accomplish things that you can't replicate, like Valmiro (wears Aculeus Suit that doesn't overwrite other creation parts) and Marienbard (wears Belle Gloves that don't clip/get overwritten by the sleeves), and/or having non-unlockable parts, like Arcturus (Algol's parts) and Ukon (chest tattoo), and also anyone who uses monster growling voice.
There's more reason to believe they won't be unlockable than to believe that they would be unlockable, however, looking at the past three games. There were only a small handful of them unlockable in SoulCalibur III, with the vast majority of story and Chronicles of the Sword characters not being unlockable, the only reason being that we got what we got was to have stand-ins for the unique styles. SoulCalibur IV didn't unlock any of them, with even more appearing in Tower of Lost Souls. Same for SoulCalibur V with the Quick Battle folks and the small handful from its story mode. We'll see. I hope so, but I'm not optimistic about it.
Edit: Added second paragraph.