The absolute peak of Avengers crossovers for me will always be the massive wave of stories that mashed them up with the MCU's own 'Sherlock' reboot. Not the Conan Doyle version, but specifically the BBC series with Benedict Cumberbatch. The mental gymnastics writers went through to fuse Sherlock's deductive genius with Tony Stark's technological brilliance, while also throwing in the John Watson/Steve Rogers dynamic, produced some incredibly smart fic. I binged a series where Sherlock was Tony's secret half-brother and Mycroft was a high-level SHIELD consultant. It wasn't just about the fights; the real tension came from Sherlock deconstructing the team's interpersonal dramas in that brutally analytical way.
A weirdly specific niche I adore is Avengers/'The Fast and the Fious' blends. Sounds ridiculous, right? But there's a subset of fics that treat Dominic Toretto's 'family' philosophy as a straight-faced, street-level counterpart to the Avengers' world-saving teamwork. I read one where the team needs to retrieve an artifact from a secure convoy, and they have to enlist Dom's crew because their driving is literally a superpower the Avengers lack. The culture clash between Steve's old-school honor and Dom's 'ride or die' code was hilarious and unexpectedly heartfelt. It shouldn't work, but in the hands of writers who love both, it becomes this celebration of found family from opposite ends of the spectrum.