Based on previous tweets, I'm pretty sure we'll have vocal songs from the following vocalists in the OST:
Gemie
Aimee Blackschleger
Eliana
mpi
Benjamin
A Gemie song will definitely be in the OST, because Sawano mentioned in
this tweet that she was singing a song for the OST of a "new anime", and explicitly stated that it wasn't for an OP or ED. There'll definitely be an mpi song, since he mentioned he was singing in
one tweet. Benjamin was in the image as well, however I'm not sure if they explicitly said he was singing. Benjamin is definitely on lyrics though. Eliana might also be there since there was mixing for their songs, as shown in
this tweet.
"Daisensei" Muroya strings are basically
confirmed to be the string orchestra in the OST,
Harutoshi Ito and
Masashi Tsubakimoto on guitars.
Shiro Sasaki is also mentioned, as well as Kenichi Tsujimoto, which is interesting as I believe this is the first time he's working with Sawano.
EDIT: Unless I missed something, it's odd that there's no mention of Hiroshi Iimuro. He's been working with Sawano on guitars for about 10 years now, surprised he hasn't been mentioned yet. Then again, it's likely he'll show up anyway.