You know, I always find it interesting when people make fics/art where Edward’s been promoted to some rank above major (which comes with the state alchemist certification). It’s interesting to think about sure, but Ed does literally nothing for the army. Mustang’s in Central putting in the effort and getting his name passed around the higher-ups and kissing ass while Ed does literally exactly none of that. He runs around the country with his brother and, begrudgingly, checks in from time to time. Not the sorta dude to be “highly considered” for promotion.
Though the one instance where he could reasonably get promoted would be in the event that 1) Mustang has some sort of opening in his sector and 2) Mustang feels like being a dick.
“Hey Fullmetal, long time no see.”
“Yeah yeah Al and I are in a rush. We wanted to leave Central an hour ago so hurry up and spit it out what’d you call me here for?”
“Oh don’t worry it’s good news. Wonderful news. You’re really making your way in the world. See Lieutenant Colonel Davis transferred recently to Southern Command–”
“Oh no.”
“–Which means there’s a spot for Lieutenant Colonel open here.”
“No no no you didn’t.”
“Longer hours, more responsibilities, sure. But better pay, and you’d get in good with some high-ranking people, assuming you know how to brown-nose.”
“I will literally kick your ass, Mustang.”
“For what? Brightening you future?”
“No really I will kick your ass if you’re being real here–”
“Is that a challenge, Lieuuuutenant Colonel Elric?”
“I swear to God Mustang if you a c t u a l l y d i d…”
And as it turns out Mustang’s completely bluffing and gave the job to some hard-working major under his command, but he couldn’t pass up he chance to see Edward’s reaction to dealing with the idea of having genuine responsibility to the Amestrian military.
Mustang knows he actually has relatively little power over Ed, because Ed is too important to the homunculi. Mustang can’t fire Ed (not that he would) or suspend him or dock his pay or dole out any punishment really for insubordination. If he tried, Ed could just completely him, and no one under Bradley’s rule would try to enforce it.
This frustrates Mustang for some time, at least until Mustang discovers he actually is in possession of a real threat against Ed: Promotion.
This confuses a lot of people, who overhear fights between Ed and Mustang that end with Mustang menacingly discussing the better healthcare benefits of Lieutenant Colonels. Or the nicer pay. Or the guaranteed apartment housing in Central.
Most soldiers start theorizing this is some kind of back-hand threat. Something like “You know Elric, you’re under consideration for Lt. Colonel and would get all these nice things. But if you’re going to be difficult, maybe we’ll give these to someone else? Think of what you’d be missing out on.”
This theory loses all of its credibility one afternoon when Mustang’s soldiers overhear a loud, heated argument between the two of them from inside Mustang’s office. It ends with the sound of Mustang slamming both his hands down on his desk, kicking his chair back, and shouting, “I SWEAR TO GOD FULLMETAL IF YOU DON’T SHUT UP I WILL PROMOTE YOU!”
Most Central soldiers just stop trying to understand alchemists at that point.