(super light spoilers for S4 up to Ep2, but figured better safe than sorry.)
This is something I have feared since like 2021, but...with all the build up to Rem's awakening, I don't think it would make sense for her to just wake up and hug Subaru and thank her hero for saving her or whatever, or for Rem and Ram to go back to their Arc 2 selves. Everytime I think about it, it doesn't make sense from a writing perspective to have this be the pay-off for her being missing for more than half the story.
My number 1 theory is that she will wake up but without her memories. Maybe the others will remember her, maybe not, but the important thing here is that she wouldn't remember Subaru. It would create a unique character conflict for Subaru to overcome since he would have saved a life but not the one he thought he did. Perhaps he would then feel shame that he is no longer living up the picture perfect hero version that the Rem of Arc 3 had for him. It would also be the first time in the story we would have had a situation where Subaru's loved ones simply...moved on from him. Not out of indifference or hatred or because he messed something up, but simply because they just...changed. I think Subaru as a character really needs an experience like that to cherish those he has. It would also be really neat to see Ram be closer to Subaru than Rem for a bit considering how S4 seems to like having Ram and Subaru show ever so slightly more affection towards one another.
Another fact supporting this is that Rem pre Gluttony was...done as a character. If she comes back as is...what do you even do with her? As a writer myself, I just can't see her being anything interesting to watch if she just wakes up fawning over Subaru all over again. She would just become...idk, a character like Felis or the man-wolf guy I am forgetting the name of who just helps out in the background every once in a while. If Rem does come back in Season 4/Arc 6, she ain't going to be the same. And maybe that's okay.
I know a LN reader is laughing his ass off at this right now LMAO