Their relationship could develop through shared hobbies. For instance, if Penny starts to show more interest in Leonard's scientific work, it can bring them closer. They can have deep conversations about his research, and Leonard can teach her new things.
Well, it's possible that Mang gets back together. But we need to know more details about the situation, like the reason for the initial split and the efforts made by both parties since then.
The story of a broken mirror being reunited usually referred to the story of two people who broke up for some reason but met again under some chance or coincidence and got back together. This kind of novel usually involved the emotional entanglement between two people and the various challenges and difficulties they faced after they got back together. Some novels would also describe the conflicts and misunderstandings between the two and how they would resolve these estrangement after they got back together.
Maybe in the fanfiction, Penny and Leonard decide to have a big party after getting married to celebrate with all their friends and family. There could be a lot of humorous moments, like Sheldon trying to take over the party planning with his own unique rules.
One common theme is their relationship growth. Fanfictions often explore how they develop from just dating to a more committed relationship. Another theme is their interactions with friends. Since they are part of a group, how their relationship affects and is affected by their friends is often a focus.
A popular story could be about how they first met and the initial attraction. Leonard was smitten with Penny right from the start, and Penny gradually saw the good in Leonard. Their first date, which was filled with nervousness and some awkward moments, was a key part of their relationship's origin. Also, their story of living together brought new challenges and joys. They had to learn to share space, deal with each other's habits, and that was an important aspect of their relationship development.