This is about predicting and NOT the usual kind of post.
Vega seems likely to return for S4 and is in every ‘main’ leak for S4. Balrog is likely for S5 also because of the pattern of releasing old SF2 bosses.
Sakura has a LOT of fans and tends to be in almost every SF game as well as some crossover games (EX, MvC2, CvS2, etc.).
Cody is highly likely as he almost made base roster and we’re STILL waiting for FF characters in a game that takes place in Metro City. He also actually has a lot of fans like Sakura.
After that, I’d guess Menat; as the successor to Rose, and since we have several new-generation characters that are replacing previous characters in SF6 (Lily for T. Hawk, Kim for Guy, Jamie for Yang/Yun, AKI for Fang, e.g.).
NO idea after that…