Obviously, the notion Chris Evans would do this is fantastical, but wouldn't that just be cool of him to do that? To lift up a female lead project, a sequel series to the first female lead MCU project. And everybody wants answers about this timeline, what that would look like. Steve returns, knows all these things about the future. Even if he truly retired, which I doubt, you can be damned sure Peggy wouldn't.

Edit: Some of your and my additional ideas I liked:

• It is the planned 3rd season, with Suza and Peggy dating and then breaking up. Steve arrives in the quantum suit sometime after that resolves/ends.
• He is stranded/out of particles, that's why he didn't arrive earlier. Maybe he gets access to more particles but chooses to stay.
• Peggy is gunna want the deets on how he returned the infinity stones. We finally get our flashback sequence.