Can't be. In his ending, Siegfried crystallized (as Soul Calibur was sustaining him from his mortal injuries in the last fight in SC3) himself and the entire tower. It'd have to be an ending combination of Hilde's, his and perhaps another (Kilik/Xianghua?) in order for him to be alive in SC5.
Thinking about it, nearly all Soul Calibur sequels take a combination of endings to canonized in the sequel. In Soul Edge, it was Sophitia's, Taki's, and Siegfried's. In Soul Calibur, it was the Trio's (Maxi, Kilik and Xianghua). In Soul Calibur 2, it was Raphael and Siegfried. In SC4, it's likely assumed to be Siegfried's and Nightmare's since SC4 happens mere months after SC3's (heck, Talim and Yun-seong never got to their ending in SC3 and were still looking for the sword). Many characters were still following the SC3 story lines in SC4 ("when along came a tower...").
I'm fine with a reboot, to be honest. I doubt there's anymore they can really do with the current cast outside of them still chasing after the sword (Mitsurugi/Yun) and leaving feelings left unrealized (Xianghua/Kilik) or still obsessed with revenge (Maxi/Setsuka). New characters means new motivations and new legacies. The new cast can be descendants of the old cast or entirely new characters as long as the styles of old return in the new game with new tweaks and twists to their game play.
How awesome would it be to see female Kilik, or a younger face on Cervantes' style? I wonder what Taki's fighting style would look like on a male character (male ninja) or Setsuka's on a fully realized samurai? And everyone's eager to see Bangoo make his appearance into the series.
I suspect the only mainstays would be characters that are probably just too cool to drop. Character's like Siegfried, Mitsurugi, Yoshimitsu (or his successor), Zasalamel, and Cervantes. The rest of the cast will probably be all new, with their fighting styles inherited from their predecessors.