I don't think they'll add Geralt now. I mean, if a Soul Calibur game comes, it will definitely not be anytime soon and it will be too far from the release of the last Witcher content. We'd have to find out the release date of SCVI (if there is any) to speculate better about the guests.
You mean the guy that replaced Mitsurugi and then returned as a Bonus in SCIII? I think he is quite cool, but Namco doesn't seem like they care too much about former Bonus Characters. Shame, 'cause I liked most of them.
You know who would be a cool add? Dante from Devil May Cry, I heard he was gonna be SCIII, but got cancelled alongside another main character due to rushing... I mean, "time constraints".