That "even if you're writing a non-canon pairing in fanfiction, you have to respect that these characters will probably still care on a basic level about their original love interest" post was inspired by a lot of things. But foremost inspired by any "Fullmetal Alchemist" fanfiction that tries to convince me that Edward Elric doesn't actually like Winry Rockbell at all.
Like, uh, no. Ed would die for Winry, so jot that down. Even if you're writing an AU where one or both of them are gay, which is fun and fine, Ed and Winry are ride or die for each other. They've known each other for so long. They trust each other so much. When they're arguing with each other or annoying the other, it's because THEY CARE. This is basic characterization for them.
I was amusing myself by thinking about an AU in which Ed's romantic interest is someone like Ling Yao, in which there's some dangerous situation where Ed has to choose between saving Winry (his sister figure in this AU) and saving Ling. I was immediately like, "Oh, he would FOR SURE pick Winry in most scenarios. Sorry, Ling." Which would suck for Ed, because he hates failing to save anyone, but is also funny to think about with Ling specifically, because I think that Ling would actually respect this decision more.
Unlike Winry, Ling is a combatant with bodyguards, and so can be trusted to handle himself in dangerous situations. (Which obviously does not make Ling a BETTER love interest for Ed than Winry, Winry doesn't need to be good at fighting, it's just a different skillset.) Ling is also a leader, someone who wants to be an emperor someday, and I think he has opinions on loyalty. Despite feeling grateful to be saved, I think Ling might think quite poorly of Ed choosing him (a relative stranger, even if he is a legit snack) over Winry (functionally a member of Ed's family / clan, a skilled specialist who is necessary to Ed's own combat capabilities), because Ed sure as shit wouldn't be considering future political advantages with Xing here, so it would be choosing a new love over family / a loyal friend. Depending on the scenario (it all depends on the specific situation for all of these cases), I also think that Ling might choose to save someone like Lan Fan or even Mei Chang over Ed in a lot of situations, especially because Ed would HATE IT SO MUCH if Ling knowingly sacrificed anyone for his sake.
Non-canon pairings and their AUs are interesting to me partially for how they interact with the existing canonical relationships and how they negotiate with canonical motivations. For some characters, romantic relationships are just not as important as familial ones or their own goals. Obviously, for Edward Elric, (depending on the specific situation at hand, of course) he's going to choose Alphonse over nearly anyone else.
