An AU where Civil War went exactly the same, with the exception of (you guessed it) Tony and Steve having once kissed/had a one-night stand. Tony therefore does not get back together with Pepper, but rather he and Steve both remain single, pining for each other. The reunion between Steve and Tony in Endgame, when Tony gets back from space, therefore goes a little differently (in a good or bad way).
Bonus if there's still a scene of Tony chewing Steve out over CW. Double bonus if there's constructive dialogue in there somewhere.