Dive into a world of creativity!
There is a version of Emilia/Subaru that makes me go absolutely feral but i'm not sure re:zero will go that way so I decided to write it out just to get it out of my system (and than accept what I'm given by a really excellent story that is just doing something different than what I want)
So in that version of events Emilia=Satella. I heard through fandom osmosis that there are two different entities in Satella (Envy and Satella). Emilia is Satella.
Second: the first time loop we see isn't Subarus first timeloop (I think that is canon because a) we never see Subaru arrive in Lugunica b) his relationship/contract with Satella is already in place) -> that can have two different reasons: a) Subaru is a reincarnation of someone Satella knew or he forgot a bunch of stuff including Satella. My feral HC combines these two
So what if a version of Subaru met Satella 400 years ago (maybe the guy who was summoned 400 years ago and brought a bunch of Japanese culture to Lugunica) but he was kind of fucked up/a bit of a fuckboy as was Satella who was at that point just completely disillusioned by the world and what if they kind of found love in each other but we're to far gone to actually make each other happy/be good for each other (or maybe just Satella was this way) and so they decided to start again both as a version who can become good. Satella sealed herself. The good part of her became Emilia and the "bad" part remained to protect Subaru. and Subaru reverted maybe to his self in the beginning of the summonings (or became reincarnated as well) and so they started again. Because Satella cannot die there's still a version out there who desperately loves Subaru and she saves him. She is the part of Emilia that Emilia has sealed because she cannot believe that she can be good if she is Satella at the same time (And Emilia was born to be happy) both Subaru and Emilia forgot her. And now Subaru hates Satella. He fears her. All that love and nowhere to go. She comes across as a crazy stalker. At the end of re:zero Emilia and Satella become one (that is how Satella is defeated) and they become able to die as well as Subaru becoming mortal as well and it's revealed that Subaru wasn't alone in remembering. There was someone else as well who remembered and struggled and tried to do better.
AND all the what ifs are canon because they are things Subaru choses at one point or the other and than Satella reverts everything back to zero because Subaru failed to become happy and good