Twilight Sparkle would likely be affected as she is often at the center of things in the MLP universe. Her actions and decisions would be crucial in any alternate ending, whether she's saving the day or facing new challenges.
The couple getting married, of course. Their relationship is the focal point of the wedding. If there's an alternate ending, their future together could be completely different. Maybe they decide not to get married after all, or they face some huge obstacle that changes their relationship dynamic.
One possible alternate ending could be that Twilight Sparkle uses her magic in a completely unexpected way to stop the wedding chaos and actually uncovers a plot that was much deeper than anyone thought, leading to a celebration of friendship rather than a wedding.
Definitely Twilight Sparkle, since she is such an important pony in the My Little Pony universe and would likely be involved in any big event like a Canterlot wedding. Fluttershy could also be a main character. Her gentle nature could add a different dynamic, perhaps she's helping with the flower arrangements or calming nervous ponies. And then the two ponies getting married should be main characters. Their love story and the events leading up to the wedding could be the focus of the fanfiction.
Twilight Sparkle is often a main character. She's the smart and studious pony, and at a Canterlot wedding, she might be involved in making sure everything goes smoothly or solving any problems that arise.
Obviously, Harry's closest friends like Hermione and Ron. They would be devastated. Hermione would probably be racked with guilt for not being able to prevent it. Ron would be angry and at the same time lost without his best friend.
Twilight Sparkle could be the main character. She was deeply involved in the Canterlot Wedding, and if there was some sort of misunderstanding or mistake on her part, she might be the one to offer an apology in the fanfiction.
Princess Celestia is also commonly included. Since it's a Canterlot wedding, as the ruler of Canterlot, she would play an important role in the organization and significance of the event. She could be shown making important decisions regarding the wedding.
Family members, if he has any in the fanfiction world, would be devastated. His mother, for instance, would have to deal with the loss of her son. She might become more involved in the affairs of the demigod world, trying to make sure that her son's sacrifice was not in vain. Also, his half - siblings, if any, would be affected. They could be inspired to step up and prove themselves worthy of Percy's sacrifice.
Peter Parker would be extremely affected too. Iron Man was like a mentor to him. His death could make Peter question his own abilities and whether he can continue to be a hero without Iron Man's guidance. He might also feel a sense of guilt, thinking that if he had been stronger or more experienced, he could have saved Iron Man.
MJ and Ned could also be part of this. If Peter's journey in an alternate ending takes him to other universes, they might be left behind at first. But then they could find a way to also get involved in multi - universe adventures, either by finding a way to travel to other universes themselves or by helping Peter from their own universe in some unexpected ways.
Well, Harry and Fleur are the stars of the 'Harry and Fleur Wedding Fanfiction'. But don't forget about the magical creatures that might be involved. Maybe a house - elf or two are helping with the preparations. Hagrid could also be there, bringing his unique charm and love for Harry. And then there are the other witches and wizards from the wizarding community. Some old professors from Hogwarts could show up, like Professor McGonagall, who would surely have some opinions on the grand wedding.