A good topic might be their friendship growth. Like how Penny helps Sheldon become more socially aware in a series of everyday life situations, such as teaching him how to interact better at parties or handle social awkwardness in a new environment.
They could be in a story where they team up to prank the other friends. This would be in line with their mischievous sides that are sometimes shown in the show, and it would be a fun and appropriate way to explore their relationship in fanfiction without crossing any inappropriate boundaries.
One appropriate topic could be their adventures in scientific experiments together. For example, they could be collaborating on a project where Penny's practical skills and Sheldon's theoretical knowledge combine in a comical yet productive way.
One common theme is the growth of their relationship. Their friendship or potential romantic relationship evolving over time, with Penny helping Sheldon become more social and Sheldon introducing Penny to new intellectual concepts.
Another great one is 'Sheldon and Penny: Unlikely Friends'. In this fanfic, it focuses on the early days of their relationship when they were just starting to get to know each other. It details their initial misunderstandings, like when Sheldon didn't understand Penny's pop - culture references and how Penny found Sheldon's scientific rants both annoying and fascinating at the same time. As the story progresses, they start to form a unique bond that is different from Sheldon's relationships with the guys.
As far as I can remember, there is no such Penny wedgie incident in The Big Bang Theory. The series was filled with different kinds of comical situations. For example, there were Sheldon's idiosyncrasies, Howard's attempts to pick up women, and Raj's selective mutism. But a wedgie incident for Penny just doesn't seem to be part of the storylines.
There could be a fanfiction where Penny's pregnancy makes Leonard super nervous. He starts freaking out about being a dad and all the responsibilities. Meanwhile, the rest of the gang tries to support them in their own clumsy ways. It's interesting to see how the characters' relationships develop during this big life change.
Penny's friendship with the whole gang is a great storyline. She is often the bridge between the nerdy guys and the more 'normal' world. For example, she helps them with social situations and they, in turn, help her with things like knowledge and different perspectives.
A lot of fanfiction could explore Penny's emotions during her pregnancy. She might be excited but also a bit scared. In the fanfic, we could see her relying more on Leonard for emotional support. And the gang could throw her a really special baby shower, filled with nerdy and fun elements, like Star Trek - themed baby clothes or a cake shaped like a scientific formula.
There might be a fanfiction where Penny is pregnant and she starts to feel really isolated because she's the only one in the group going through this. She misses her old life a bit, and the story could be about how she overcomes these feelings with the support of her friends and Leonard. It's very emotional as it deals with her internal struggles.
One positive change was her growing acceptance of the guys' geeky interests. She started to enjoy things like comic - con and science - related stuff. Another was her becoming more responsible, like when she got a better job and started to manage her money better.
One great story could be about Sheldon and Leonard having a 'bro day'. They might go to a science convention together, and along the way, Sheldon starts to understand Leonard's need for a bit more social interaction. He tries his best to be more 'normal' in public, like not correcting every little scientific inaccuracy he hears. This shows their growth in their relationship.
The significance of Penny's belly dance was mainly to add some fun and spice to the show. It showed her as a carefree character.