Because it's highly likely that The Big Mac Question means Sugarmac wedding, this is who I guess are gonna be the positions (with Big Mac and Sugar Belle's positions given facts).
Mayor Mare always officiates the weddings, but Twilight is a possibility.
For the Bridesmaids, I chose Night Glider cause she's basically Sugar Belle's closest female friend, Applejack cause it's her brother she's marrying, and Starlight but I'm sure she wouldn't be comfortable given by their past. I dunno who should be the Mare of Honor though — so I'll let you guys and gals pick! ;)
Spike of course should be the Best Man because he and Big Mac are implied to be the best of friends judging by how much time they spend together on Guy's Night — plus we don't see enough of their friendship in the series! There's a possibility of Discord being jealous though!
I thought the Tree that the Apple parents made would be the perfect spot for them to get married, because I'm sure Big Mac would want his parents to be there for the wedding! Oh the feels, Applegeek would lose it! XD