I've probably forgotten a lot of characters, but I can only think of three possibilities (in the order I think is most likely):
1. Shou bun kun - He was a well-respected military leader before he retired. He was recognized by both Ouki and Kyou of the former Six Great Generals.
2. Ju Ko Ou - The beast-level strategist from Juuko who was captured by Tou. He had no allegiance to Chu, as he said, and if he were to agree with Ei Sei's plan to unite China, he'd be a solid choice.
3. Riboku - He's been too quiet since he had to flee. If he fled Zhao with Ba Nan Ji and came to Qin, Ei Sei would be an idiot not to bring him on board. If Riboku decided that the best way to protect Zhao was to have it join Qin, he'd be a solid ally. Plus that's a wonderfully dramatic choice.
If it's Heki or Ouhon, though, I will be annoyed.