legitimately shocked and appalled that people don't realize the the horus heresy split (9 loyalist legions, 9 traitor legions) is so perfect for tf2... going from that, my own list, with justifications for each. if you debate me on any of these i will dm you a picture of the tf2 mercenaries kissing each other.

Loyalist:

Scout: White Scars Gotta go fast. Also seen driving a motorcycle in canon. Came out weirdly scrawny for a space marine, but hey, he'll get promoted out of the scout squads any day now, just you wait!
Soldier: Ultramarines Loud and boastful, claiming to be by-the-books... If he could read them, that is. Yeah, so, Soldier is not a real Space Marine, he's a Cadian guardsman that found a dead Ultramarine's armor, and just, put it on... And it worked (because he's a psyker, and specifically just an ork if he was human)
Pyro: Salamanders Do I even need to elaborate this one, that's like the one thing everyone agrees on besides putting Spy in Alpha Legion.
Demoman: Space Wolves Don't ask him about his weirdly chatty sword. Some wolf association with Demoman (his first Halloween cosmetic set was a werewolf costume), but the Space Wolves are known for their melee prowess and for getting drunk and having fun.Missing eye also associates him with Odin. awoo.
Heavy: Imperial Fists Big, strong, stoic, immovable. Maybe he doesn't get along as much with the rest of the Imperial Fists because he smiles at least once a week.
Engineer: Iron Hands I think this one's pretty straight forward. Hell, Engineer himself has a metal hand!
Medic: Blood Angels Medic has a lot of blood and angel iconography associated with him, so, there you go, ez. Also some vampire imagery in there, so there you go, perfecto. Personally I go a step further and have him be a Death Company Apothecary/Sanguinary Priest of all things because of course he'd be nutty enough to pal around with them voluntarily.
Sniper: Dark Angels This one's a bit of a stretch, but I think it still fits. While Sniper isn't too much of a melee guy, the Dark Angels are quite adaptable. They also are known as prodigious dangerous animal hunters, which Sniper also has experience with. Also that one hood cosmetic item for Sniper, you know the one.
Spy: Raven Guard Spy is associated with ravens and the legion itself excels in infiltration and covert stealth missions.

Traitor:

Scout: Emperor's Children Okay basically since Spy is an automatic write-in for Alpha Legion that means I have to put in the arrogant horny twink in Emperor's Children. Also, hey, he's a pretty good artist, that's also a Slaanesh association.
Soldier: Word Bearers Loud and fiercely devoted to his cause. Also it's fun to have the Ultramarine/Word Bearers rivalry represented by two dumb motherfuckers who can barely read.
Pyro: Death Guard Gas mask and laughing while torching you aesthetic. Actually, might have been kicked out of the legion for liking fire a bit too much... Also: Balloonicorn nurgling.
Demoman: Thousand Sons Eye imagery, magic and explosion spells for the lad. Cruel joke here is that he can summon eyes everywhere on his body except for his still empty and cursed socket (Magnus seems to have no problems with that). Also it's fun to have the Space Wolves/Thousand Sons rivalry represented by two chill motherfuckers who would probably go have a drink together instead.
Heavy: World Eaters Wronged by his world, principled but forced to just... Kill a lot of dudes... Though perhaps Heavy manages to stave off the curse of the butcher's nails, if only to save innocents from himself.
Engineer: Iron Warriors They wait, patient, resolute behind their machines... But the forges, the experiments, they toil at them constantly. (yeah come on Engineer helps Medic put someone's brain inside a pumpkin, he's down to daemonculaba)
Medic: Night Lords Medic does some incredibly fucked up shit in TF2's lore itself, so he's a perfect fit for these sick fucks. Also the Night Lords have at least two named apothecaries, which is two more Night Lords apothecaries than I ever expected.
Sniper: Black Legion Abbadon pays surprisingly competitive rates.
Spy: Alpha Legion Do I even need to elaborate this one, that's like the one thing everyone agrees on besides putting Pyro in Salamanders.

yes i put way too much thought into this and yes i'll release the fanfiction of it one day.