If the story reaches the point where Siegfried reclaims his mind from Soul Edge/Nightmare, Raphael is essential; he was the key figure in that happening and so far the story beats match up entirely with what was known before. If not, I guess his chances are way lower, since his story arc was pretty insulated until that one moment of importance and he had basically nothing to do with anyone else in the cast until III, when SE's corruption took hold (a direct result of stabbing its eye) and he had Kilik/Amy to play off.
Somewhat true, in SoulCalibur II, this is what happens, but not in SoulCalibur, and I don't really blame you for getting it mixed up, because the stories of SoulCalibur and SoulCalibur II are so similar that you might even call them clones. But SoulCalibur VI appears to be covering events of SoulCalibur leading up to SoulCalibur II, but not the events from SoulCalibur II itself.
In SoulCalibur, Kilik, Xianghua, and Maxi confront Nightmare, Inferno is unleashed, Xianghua defeats Inferno with Soul Calibur, and Siegfried is momentarily freed, but still has nightmares about his experiences and doesn't completely regain his sanity. He lapses every now and then, and eventually loses himself completely once again, where he becomes Nightmare in SoulCalibur II.
During this time period, we have Raphael's situation with his family, being betrayed, him having to escape, finding Amy, and seeking asylum while things die down, where Amy helps him because she hates soldiers, and Raphael is being pursued by them. Raphael isn't interacting with the swords yet, but he does read about Soul Edge, which intrigues him and he wonders if that is what motivated the nobles, and eventually leads him to find Nightmare in SoulCalibur II.
In SoulCalibur II, Kilik, Xianghua, and Maxi confront Nightmare, Inferno is unleashed, Xianghua defeats Inferno with Soul Calibur, but Siegfried isn't freed this time, because he's almost fully lost. Raphael enters the scene and stabs Soul Edge in the eye, releasing the sword's hold on Siegfried, but then the Soul Edge shards contaminate him, and he's pulled to safety by Amy and healed, albeit cursing Amy in the process. Soul Calibur is then fused with Soul Edge to form the Soul Embrace to keep the sword in check until Zasalamel happens in SoulCalibur III.
