In between missions and battles that drain even a god - one who is constantly hyper - Steve and Natasha do things. It could be that, but it could also be knitting, which, yes, they did once, and, no, I'm not lying.
They don't interact with each other much, just during these quiet moments that go completely unnoticed by the rest of their team.
The first time, they spar. The chitauri invasion had just ravaged the city like some drunk ravages a poor hooker. They hit and kick and miss and groan and growl. Only a few, choice shots hit their targets, and Steve can't help but be impressed. Even at her side during most battles, Steve never sees Natasha do anything like this. Usually, she spins, twirls, kung-fus and twists. Now, she kicks, circles, judos and boxes.
Natasha wins the first few rounds, but Steve isn't detered, and he fights. He wins one, two, even three rounds. Steve decides that he prefers sparing to the punching bag.
The sixteenth time, they drink. Steve drinks mostly Budweiser, whereas Natasha goes for the Van Gogh and Grey Goose vodkas.
He doesn't get drunk, per say, but he gets dizzy, and that's enough for him. Natasha gets wasted, draping herself around Steve, and telling him all about the unicorns that prance around in Neverland. Bruce catches them like that, Steve desperately trying to get Natasha to let him pick her up and drag her back to her room, and Natasha acting like a fish, flopping around and such.
Bruce doesn't say anything, though he glances at the empty bottles that litter the counter with disdain, and leaves.
It takes a while, but Steve ferries Natasha back to her room, and then doesn't leave until morning.
(Her pleading for him to stay is what caught him. He never says no to a woman.)
The thirty-eighth time, they write letters to kids who have sent them fan mail. They ignore the marriage proposals, the bills, the spam, the offers, the ones that smell of perfume or cologne, and focus on the ones from kids.
Dear Mrs. Bruington's 3rd Grade Class, one from Steve starts. Dear Lilly Colt, another begins.
They do this for hours, wishing happy birthdays, thanking those who sent pictures they drew, telling kids that they should not try anything they've seen the Avengers do, asking how they are doing, and assuring that they are fine, that their injuries have healed.
They're happy to do it in silence.
The fortieth time, they do that.
Steve's a lot better at it than any of the others thought.
Natasha's happy to prove Tony wrong.
The fifty-sixth time, Steve draws her.
Natasha's drawn as a comic book hero and as an escaped convict, as an angel and as a devil, laid out in his bed and on the field and with kids and with Clint on the shooting range and dressed as a pin-up girl and as a Hulk, looking only somewhat like Jennifer (Walters, Bruce's cousin and resident She-Hulk).
She loves all of them and hangs them up in her room. (And, she kisses him on the cheek, too.)
The sixty-eigth time, he tells her that he loves her and promptly passes out.
Natasha has no other option than to drag him to Bruce.
The eightieth time, Steve cooks for her and she bakes for him.
Admittedly, he is the better cook and baker, but still.
The last time, they're in bed. The room fills with gas, the dorrs are locked and it takes the rest of the team too long to open the door.
Every last one of them are surprised to find that the two are together.
There's a private wake - the press is told that they died, but there will be no funeral, because there already was one and they - the press - weren't invited.
Tony finds all of the files of what the pair did and gets drunk.
Clint stays on the range until his fingers bleed and blister.
Thor and Hulk constantly fight.
And that's the end of the Avengers.