Ok, so this might be a tad crazy and I'm not even sure if it works in writing rather than as a movie, but imagine the following.
The fic starts with the regular "welcoming feast" (preferably HBP). Fleur is announced as defense teacher, things look normal, but as Harry leaves, he catches a glimpse of Diary Riddle. Just for a moment, and when he looks again, he's gone.
Harry thinks it's just his mind playing tricks on him, goes to bed, all is well.
After the first defence lesson, Fleur asks him to stay back and arranges extra lessons. Those are somewhat similar in concept to occlumancy lessons with Snape, only without the abuse. A lot about projecting your magic and an "intuitive" control of it.
However, afterwards, Harry catches another glimpse of Riddle. So he rushes to his dorm, grabs the map and finds... Nothing. Well, maybe one of the Slytherins got a new haircut or something, he's clearly just being paranoid.
So Harry continues to have the lesson with Fleur, but they are pretty much the only noteworthy thing which happens. No crazy assassination plots, no creature rampages, the only unusual thing is that Harry continues to see glimpses of Riddle, but before he can ask anyone for a second opinion ("Hey Hermione, who's that guy over there who looks like teenage Voldemort"?), he's gone.
Harry continues to grow closer to Fleur (who has no ongoing relationship with Bill, this is not supposed to be a HBR ripoff) because the lessons get quite intimate, especially as they dissect the lessons Snape has taught him (and the general mind reading/link).
There are more and more inconsistencies. Some of the first years remind Harry of the previous batch, not that he pays much attention to them. Alicia Spinnet replaces Snape as potions teacher, Fleur mercilessly teases Harry for having had a crush on her even if he has never acted on it and moved on.
Also more general weirdness which still could be explained with Hogwarts being Hogwarts (random holes in the stairs level). Harry also can't remember the summer at all and there's a lot of fog around Hogwarts.
Eventually, after more lessons with Fleur, Harry manages to confront Voldemort and it develops into a big fight which Harry barely wins after applying the lessons about magic which Fleur had taught him.
Now here's the big twist, Harry wakes up in St Mungo's, it's only around his 16th birthday, he has been in a coma since the DoM and Voldemort's possession attempt. After Harry showed no signs of waking up, Dumbledore consulted experts but they could do nothing. So he hit research mode and found something about Veela magic and linked fates (I want to avoid tropes like life debts and Veela bonds, the actions of Fleur should be completely voluntary), which have a certain mind-link interaction (requires initiation and is only active while maintained, no passive/permanent bond). Fleur agreed to go through with it, there can be a certain sense of "he saved me/my sister, that's the least I can do", but it should not be an obligation.
So Harry wakes up after what he thought was half a year but actually just a few hours (plus a month of previous coma without Fleur involved), and it is pretty shattering since none of what he went through has happened. And well, given that he fell in love...
I'd end the story with Fleur talking to Harry and telling him that just because things happened in his head does not mean that they were not real. She was actually in there with him and not just his brain projecting stuff. And all of her actions were actually her doing them. So happy, fluffy ending