Weirdly, my memory of Tom and Tord fics is that they're almost all about invasions. And I don't mean that metaphorically. I'm not sure how it started, but half the plots hinge on Tord's army from 'The End' coming back, but this time he's got... mixed feelings about capturing Tom specifically. The other major route is the reverse: a post-canon redemption where Tord slinks back to the flat alone, half-dead and expecting a fight, and Tom has to decide whether to slam the door or patch him up. Both scenarios exploit that delicious push-pull between their inherent rivalry and the weird, grudging domesticity they had. Tord's a control freak with a god complex, Tom's a chaotic slob with a heart under all that sarcasm—the tension writes itself.
Also, the 'bodyguard' trope pops up a lot. Tord, now a powerful figure, 'requisitions' Tom as a personal guard, claiming it's for security while obviously enjoying having his old rival under his thumb. It's a power dynamic playground. You'll find a ton of hurt/comfort fics stemming from that, where one of them gets injured (usually Tord, because his plans backfire) and the other is forced into a caretaker role. It breaks down the walls in a way direct conversation never could in the show.
Honestly, the best ones I've read don't even involve the Red Army. They're just mundane AUs where that same aggressive energy is channeled into, like, rival bakery owners or competing architects. The core is always that explosive combination of irritation and fascination.