S3 AU where Ed thinks Oswald may be jealous of Isabella and Ed because he wants a relationship of his own that's like theirs and ends up making a wish on something for Oswald to "have what Ed and Isabella have with each other". The next day Oswald doesn't show up for something he and Ed had planned, Ed panics after it turns to the next day, only for Oswald to walk in looking a bit dumbstruck. When Ed asks what happened Oswald says "Nothing," but he's clearly troubled. Ed later catches Oswald asking a picture of Gertrud if 'running to your one true love' is supposed to have so many false finish lines and branching paths.
Basically the wish comes true, too accurately. Oswald can't seem to keep Ed in mind for anything anymore, and Ed is concerned when he meets the person who Oswald is suddenly smitten with. It's too fast, the person has a creepy obsession with Oswald's past murders and tragic history in Arkham (unlike Ed's own totally reasonable interest in the same subject, totally reasonable yeah), Oswald keeps ignoring his plans in favor of this person which is just so unlike him...
Isabella, though, is delighted. Even lightly, gently, scolds Ed for not being happy for Oswald. "It's such a rare thing," she breathes wistfully. "To find what we have."
It is. It is! ... Which makes this even more odd. In fact, now that Isabella has pointed it out, the parallels are much too... exact.
Clearly, this person was stalking Oswald for quite some time and, upon seeing Oswald crave what Ed and Isabella shared, decided to manufacture the same kind of relationship to get into Oswald's life past the usual safeguarding.
Unsure how to incorporate a similar thing to Isabella making herself look even more like Kristen into the way this person acts for Oswald... the obvious is of course the person trying to look lore like Ed, but that doesn't carry the same kind of PRSD-inducing element. Could lean into Oswald's odd thing with his mother and have them try to invoke her in some way (recently realized that "Anything for you" didn't just hit Oswald like it did for the loyalty element, but that st the start of the episode Oswald outright says "My mother told me every night she would do anything for me" so. There's basis there) but that's a territory of fucked up I don't wanna venture into, frankly. Dammit Oswald, why can't you have a dead ex?
Anyway, I'll work that out later, whatever that catalyst ends up being (including a rejected breakup slap or the like since Isabella did that too) is what makes Ed realize his wish somehow came true. And how much he hates seeing Oswald in this relationship, but he knows it's more than just discomfort over the way it exposes how his relationship with Isabella is Not Good.
I'm unsure on the ending... but I know people end up dead. Just figuring out who kills who and how and why.