Well, a great plot could be Twilight Sparkle as an Alicorn going on a journey to discover the origin of Alicorn magic. This could involve her meeting ancient Alicorn spirits and learning secrets about her own powers. Or perhaps she gets caught in a time - loop where she has to repeatedly use her Alicorn powers to break free and figure out what's causing it. It could also be interesting if she has to teach other ponies how to use Alicorn - related magic.
One idea could be that Twilight's transformation into an Alicorn brings new responsibilities. She has to learn how to use her enhanced magic powers to protect Equestria from a new and powerful threat that normal magic can't handle. For example, a dark force from a parallel dimension starts seeping into Equestria, and Twilight, with her Alicorn abilities, is the only one who can close the portals.
One great 'twilight sparkle alicorn fanfiction' is 'The Alicorn's Ascent'. It shows Twilight Sparkle's journey as she fully embraces her Alicorn nature and has to deal with new responsibilities. The story has a great plot with interesting character developments.
One plot idea could be Twilight Sparkle getting cold feet before the wedding and her friends having to reassure her. Another could be a magical mishap during the ceremony that causes chaos but in the end makes the wedding even more special. Maybe an old enemy tries to disrupt the wedding but Twilight's love for her partner overcomes all obstacles.
One idea could be that Prince Blueblood has to team up with Twilight Sparkle to solve a mystery in Canterlot. Maybe a series of magical artifacts are disappearing and their combined knowledge and skills are needed to figure out who is behind it.
A plot could involve a race between Thomas and one of Twilight's flying friends. Twilight has to referee and make sure it's a fair race. Along the way, they discover that there are some dark forces trying to disrupt the race, and they have to work together to stop them and ensure the race is completed successfully.
One plot idea could be them going on an adventure together to discover a new type of magic in Equestria. Another could be Timber helping Twilight with a difficult spell she's been trying to master.
A plot could involve Flash Sentry and Twilight Sparkle teaming up to organize a magical event in Equestria. However, things go wrong when a jealous pony tries to sabotage their plans. They must then work together to overcome the obstacles, find the culprit, and still make the event a success. This would show their problem - solving skills and how their different personalities complement each other.
One interesting plot could be that Twilight's dragon transformation is part of a test by a powerful magical being. In her dragon form, she has to complete a series of challenges in a dangerous dragon - inhabited land. The challenges could be related to controlling her new form, using her dragon - specific powers, and also learning about the history of dragons. This would not only test her physically but also her mental and emotional strength.
One idea could be that Trixie and Twilight Sparkle team up to solve a mystery in Equestria. Maybe a powerful magical artifact has gone missing and their combined knowledge of magic is needed to find it.
One idea could be that the twin sister has a different magical ability. For example, while Twilight Sparkle is good at spells related to knowledge and friendship, her sister could be excellent at magic related to nature. This would create a contrast and also opportunities for them to learn from each other. Another plot could be that the twin sister gets into trouble in a different pony world and Twilight has to go rescue her, leading to all kinds of adventures in unknown places.