All of them, Talim, Zas, and Yun-Seong/Hwang...
...and Amy, and Cassie, and all the SCIII CaS styles, and all the non-mimic guest characters that have ever been in the series. Or at least their styles.
I'm fine with characters themselves disappearing or being replaced due to story reasons, but at least leave in their fighting styles for CaS. I believe that a good developer doesn't remove content between sequels, at least not for something like "we had to rush to meet a deadline. You still have to pay $60 for this barely complete game, though."