One trope I've seen a ton is the 'post-season trauma support' angle. After everything that goes down in Hawkins, writers love putting Steve and Eddie together because, honestly, who else gets it? They have this shared understanding of the Upside Down that even the Party can't fully touch. A lot of fics start with Steve showing up at Eddie's trailer to check on him, or Eddie seeking Steve out when the nightmares get bad, and it just spirals from there.
Then there's the classic 'babysitting co-parents' scenario. Steve's already got his little herd of kids, and adding Eddie into the mix as the 'cool uncle' who teaches them D&D and guitar is a natural fit. The dynamic shifts from Steve being the responsible one to Eddie stepping up in his own chaotic way, and seeing them figure out how to work as a team is usually the heart of it.
Also, you can't ignore the 'opposites attract' trope played straight. The former king of Hawkins High and the resident metalhead freak. Fics love exploring the tension of their high school personas colliding as adults, with Steve realizing Eddie isn't just the loudmouth he thought, and Eddie seeing Steve's loyalty and care underneath the hairspray. It's all about the mutual, grudging respect turning into something else.
I'm also a sucker for the 'Steddie band AU,' though that's more of a setting than a trope. Eddie's the frontman, Steve's maybe the bassist who joined on a whim, or the roadie, or the manager. The chemistry of creating something together, plus the tour bus proximity, just works.