Obviously we'll have Subaru, Emilia, and Rem. The real question is who the 4th (or 5th) will be, cause we got a damn good lineup. Ram, Roswaal, Echidna, Elsa, Reinhard, Crusch, Garfiel/Frederica, Felis (monkaW). Personally I'm fine with any of them, with a preference towards Echidna and Crusch.
Edit: Wilhelm also cause Charles could use a friend
Like Kajimoto said, it would be more surprising it if's not Subaru, Rem, Ram, and/or Emilia. The chances they would add another unit outside of those four would be the more surprising route.
I would expect that if it's modeled after GG (meaning 1 free unit, 2 ltd) I would expect Emilia as the free unit, and Rem and Ram as the ltds. I think it's in their best interest to not make Subaru a playable unit. The extent to which they would have to invent a skill set for him would, IMO, be too much unless they make him a comedic relief character in the mold of Kizuna Ai. That is part of the reason I was never high on Re:Zero being the collab, since you are forced into decisions that either don't fit E7's overall atmosphere (overly comedic skill schtick), or require unusual choices for the IP characters (what actual powers do you give him to match up w/ even someone like Enott?).
I'd love to have Emilia and Roswaal or demon ver of Rem and Ram. Elsa seems really solid as a Thief too! Or or BEAKO? Tbh I'm just looking forward to seeing how they design the S3, that's all I want. I wasn't aware that Re:Zero are known to do many collabs, but I'm genuinely looking forward to this one since its coming to E7~