Removing Sonya and JC would be like Street Fighter dropping Chun Li & Blanka - oh wait they just did some of that...
I don't see a need to drop any, they're all solid characters with a good diversity of moves and very popular in terms of use.
It's good for fighting games to keep their classic core, that appeals to long term fans that helped make the games what they are and allows for deeper characterisation and stories to occur.
As far as new characters, Cassie is great, she has similarities to both, but plays quite differently from either and her personality is very well created for such a new character. She has some of the best match intro quotes by far and there's some interesting routes the story could go with her. She could do with some more interesting costume designs though.
As a sidenote, I don't play with any of the Cage crew, so this isn't fan bias.
Fighting games need to change their cast up every now and then, but there's a core they should keep, it causes too much upset that detracts from the series if they drop major players.