As someone who has just watched the Old Guard and used to be a pretty big Captain America fan, the most hilarious crossover would be one where the four of them had helped the Howling Commandos in the war and also forget to watch the news and after walking into Steve on the street one day spend an inordinate amount of time trying to figure out if they just completely missed a new immortal somehow.

He’d have been allowed out and about for like a week before he runs into Nicky and Joe and automatically greets them since they know each other and his momma raised a polite young man thank you very much, the two just awkwardly nod along and try to run away as discreetly as possible before Steve has the time to think about is he fact that logically they should be old men by now.

Andy is trying to decide if the no doubles rule to jobs applies when the dude is supposed to be dead and may or may not be an immortal since none of them can agree. Booker finds out they found him stuck in the ice and the argument just gets more convoluted. 

All of them talking at once while staring up at the ceiling in a fit of existential crisis

Andy: Maybe the ice didn’t kill him?

Joe: What if we didn’t have dreams because we’d already met him?

Nicky: How do we know he hasn’t been stuck dying in the ice and we just can’t tell because he was trapped in the ice?

Nile, just sitting in the back of the room getting louder by the minute as they ignore her: WAIT YOU GUYS KNOW CAPTAIN AMERICA????

Two years later Bucky enters the scene and boy howdy, they’re just popping up like daisies!