Imagine if Harry wanted to ask Fleur out, but had no idea how. So after a bit of consideration, he ends up asking none other than Lavender Brown. At first, Lavender just gives him advice, but after the first date she really gets into the whole thing and starts her own plots to get Harry and Fleur together while also (really obviously) wingmanning for Harry.
The story could fast forward between important milestones in the Harry/Fleur relationship and somehow, Lavender always has her fingers in there. It could end with either Lavender as Fleur's bridesmaid or with Lavender being made godmother of their first child.