One idea could be that the twin brother has a different magical specialty. For example, while Twilight Sparkle is great at magic related to friendship and harmony, her brother could be more attuned to magic of the elements in a different way, like having the power to manipulate natural elements more directly. Maybe he could use his magic to help protect Ponyville from a new, different kind of threat that Twilight's magic isn't as effective against.
First, you can start with his appearance. He could look similar to Twilight but have some distinct features, like a different color in his mane or tail. In terms of personality, he might be more outgoing than Twilight. For example, he could be the type to easily make friends with everypony he meets. This would contrast with Twilight who can be a bit more reserved in new social situations.
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 Twilight Sparkle's daughter discovers she has a unique magic ability that even Twilight doesn't fully understand. This could lead to her going on a journey to find out more about it, meeting new characters along the way.
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 interesting storyline could be Twilight Sparkle's little brother getting involved in her magic studies. He might accidentally cause some chaos in her study area with his own budding magical abilities. For example, he could make all the spell books float around in a disorganized mess while trying to help Twilight organize them.
One idea could be that Ash's twin brother was separated at birth and raised in a different region. He might have a different approach to training Pokemon, perhaps focusing more on strategy than Ash's instinctive style. Another plot could involve the twin having a special bond with a rare or legendary Pokemon from the start, which sets him on a unique journey.
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.