In real life wars do not end just because someone says it is over, that's not how it works and it is this basic premise we can use to build upon. There exists many great fics that bring Harry and Fleur together however very few if any that explore what comes after. It is this element which could give lots of creative room and writing freedom and inspired the following plot.
After the war Harry and Fleur settle down into their respective roles and start a family. While this is important it takes more of a backstage as we focus on specifically their children or child depending on how deep you want to go on character development.
As I said in the beginning wars don't magically end just because someone says it did or you killed the general. That's not how it works and it is this which is the focal reason for the plot that will happen.
Death eaters were not just confined to the British Isles. In fact they had support from all over Europe and elsewhere. Despite Voldemort begin defeated there are still many followers who want to get even or change the outcome of the war. How this happens can be up to you either in the present time or my own favorite one that could lead to all sorts of interesting paths - time travel.
But it would be far too cliche to just have a bunch of death eaters go back in time to prevent Harry from being born by killing his parents or something of the like. No, instead due to the plot device there is either a limitation that prevents them from doing that or an accident that prevents them from going that far back. Instead they arrive at some point during Harry's Hogwarts years either at or before the tri-wizard tournament. Determined still in their mission and being unable to directly kill Harry themselves they instead attempt to or get in contact with death eaters and begin funneling information that could change the entire outcome of the war.
However what they don't know is that Harry and Fleurs child/children found out these death eaters went back in time and through whatever means you the author chose followed them back to stop them. However to make it interesting instead of revealing themselves they must stay hidden working in the shadows until something happens at some point where Harry and Fleur become aware over time that their child (who they do not know is theirs) is actually theirs or the child/children reveals they are from the future and what they are there to do.