We knew that Ishida would be with the Sternritter, but I don’t think anybody even remotely suspected that Yhwach would name him as his successor. Ishida is a newcomer and doesn’t seem all that strong, making him a less than obvious choice to be the next emperor of the quincies. Likewise, the Sternritter are also quite shocked at some random kid that they’ve never seen being picked as their next leader. It doesn’t help that Yhwach squashes the dissenting voices immediately and refuses to offer an explanation. That said though, I think he has a good reason for selecting Ishida, and his choice implies that he has insane hidden potential that only needs to be unlocked. Yhwach mentions that Ishida is the sole quincy survivor in this world, which was confusing since Ryuuken is still alive; in addition, his statement seems to imply that the Sternritter and the rest of the army are somehow different or not genuine. Even more strange is the fact that Ishida is mixed-blood (pretty sure his mother was not a pure quincy) so it’s not clear what Yhwach’s rationalization is.
The Sternritter are quite angry at this development as they expected Yhwach’s second in command Jugram Haschwalth to be next king. However despite their arguing among themselves, I doubt they’ll actually rebel, and once Ishida (who now has the letter A) displays his powers they will probably grow to respect him. He had better be ridiculously powerful; and for once hopefully he’ll be able to give Ichigo a run for his money. Ishida has been a pretty static character throughout the bulk of the series, so maybe now it’s finally his time to shine. Of course it’s still possible Ishida is acting as a double agent even if nobody else knows, but it’ll be interesting if he turns out to be the next super villain.