this is gonna be sacrilege but…
i think if marlene had survived and the war had ended, she and sirius would have been that fiery couple, the kind no one would ever denied were deeply in love with each other but, at some point, would have gotten separated.
after a while, they both would have moved on, found new partners (never got kids though), saw each other occasionally, mainly for harry’s birthdays.
they would have become that couple, even though they were no longer together, that every one talked about when they weren’t present.
no one ever understood why they broke up.
their friends are still waiting for them to get back together.