One possible plot could be that it's an alternate universe where the war ended differently. Harry, in his quest for peace and to end the cycle of hatred, decides to marry Rabastan Lestrange as a political move. He hopes to reform the pure - blood supremacist ideas within the Lestrange family. They might start off as strangers, but gradually learn about each other's pasts and find common ground in their losses. Harry could use his influence to bring Rabastan into the light side, and together they could work towards rebuilding the wizarding world in a more inclusive way.
Perhaps in this fanfiction, a powerful magical curse forces Harry to marry Rabastan Lestrange. At first, they both resist the situation vehemently. But as time passes, they discover that they have been misjudging each other. Rabastan might show Harry a side of the pure - blood culture that isn't all about supremacy, and Harry might introduce Rabastan to the Muggle - born world and the values of equality. Their relationship could then develop into a love story, despite the odds and the initial shock of the wizarding community.