Yoshida did something a few months back about expecting some main characters to die during the 4.0 expansion (across all the patches). Plus Estinien returning? I can totally see them killing off Y'shtola and then having Estinien join the Scions. Afterall Krile replaced Minfilia and Alisae replaced Papalymo (I'm talking numbers here).

I can see them slowly removing the original Scions one by one and building a new Scions of the Seventh dawn.