The hooded star of Assassin's Creed 2 and Brotherhood, Ezio Auditore, is strongly rumoured to be a playable character in Soulcalibur 5.
Joystiq was anonymously sent a picture from "a recent fighting tournament" of Ezio on the character select screen in Soulcalibur 5.
Soulcalibur has a history of guest-fighter cameos. Soulcalibur 4 famously featured Darth Vader and Yoda.
Soulcalibur 5 will have at least 26 characters, according to Project Soul.