It is certainly an interesting idea - Suzaku certainly "changed the course of history" within the Code Geass universe, and thus would almost definitely qualify as a wild card - but would the "White Knight" actually use the power of the wild card properly? Lelouch, for all of his lack of trust in others and need to control everything around him, plays more as a "Persona protagonist" because he would actually "do whatever was necessary to hone his newfound power," even something as laughable as exploiting the power of friendship (actual words from Wings of Rebellion). Pre-SAZ Incident Suzaku, on the other hand, is very much a lone wolf. Not unlike Emiya Shirou from the Fate series, he's the type to throw himself in front of a bullet not because of any personal feelings for the other person, but because he sees his life worthless compared to everyone around him, and thus simply wants to save them. There's a reason that, even in his two meaningful relationships, he still acts almost subservient to both Lelouch and Euphemia. I could very well picture the "White Knight" doing everything he could to keep others away from the Metaverse, choosing to shoulder all of the burdens alone rather than relying on friends to support him - and that makes him really tough to picture as the main character in a Persona story. Not impossible, of course, but definitely difficult.