So, the thing I keep hearing the most about these contest from writers is: "how do I get them together?"
well, I've brainstormed a few little scenarios to get these two together in a believable manner.

Fleur comes to Ponyville:

1.) Fleur is in town to model for Rarity's newest fashion line; along the way she meets a certain big red stallion.

2.) Fleur has comes to Ponyville on a mission to meet the legendary mystery model: Fluttershy; along the way she meets a certain big red stallion.

3.) Fleur is sick of every attractive stallion she meets being either gay or married, or both. She decides that to go to the place with the least number of gay and married stallions in all of Equestria: Ponyville. Little does she know, this is because Ponyville has the least number of stallions in general in all of Equestria.

I really couldn't come up with any plausible reasons for Big Macintosh to come to Canterlot. But anyways, hope these help.
Good luck!

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I was mulling over an idea revolving around Big Mac winning the Equestrian Lottery or otherwise coming into a large sum of money. Kind of a "you find out who your friends are" sort of stories. Fleur would get a line something like this:

"So what if we only met because of the money? You coming into my life was the best thing that has ever happened to me, Big McIntosh, and I'm not going to let a few bits drive us apart as easily as that."

Remember when Applejack had that stand at the Gala? Imagine that Fleur sampled her wares and wants to track down the origins of the delectable delicacies. The road to a mare's heart is through her stomach, after all.

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Fillies love a gentlecolt who can cook... wait until she tries his apple fritters.

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How acceptable are AUs?

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AUs are 100% acceptable. Read below to see what I mean.

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I've been brainstorming this for a while now, and have concocted my own special scenario based on a previous story and my No Heroes universe. The story I intend to write will actually be a sequel to my October entry! This may mean a return to the spooky element, though, so I'm not expecting to actually win with this concept. Still, it's an entry.

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WRITE IT WRITE IT WRITE IT!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!