Oohinata probably expected Chitanda to have brought something up already, and she confirmed at Houtarou's home that she likely hasn't forgotten it and must be withholding it intentionally.
I think he's said before that the priestess ended up as an essential hero in his first playthrough, so it's probably not her, unless we're having a redemption+recruitment chapter next.
I love this arc. I don't care how long it will take, we're getting to see all these characters get better. And showing improvement, no matter how small, is what world trigger has been about from the start.