There are fan fictions that bring in other characters from the Arrowverse in unique ways. Maybe a story where Oliver and Felicity team up with other heroes to fight a new and powerful enemy, and during the process, their relationship evolves in unexpected ways. Also, some stories might explore their pasts more, like how they were before they met and how those past experiences shape their relationship.
One common plot idea is exploring their life after the events of the show. For example, how they handle raising a family while still being involved in the vigilante world. Another idea could be alternate universes where they meet under different circumstances, like Felicity being a superhero herself and Oliver being a tech genius who helps her.
Some fan fictions focus on their relationship in more depth. For instance, dealing with the challenges they faced during their relationship in the show but in a more detailed and emotional way. There are also stories where they get separated and have to find their way back to each other through various adventures and self - discovery.
One idea could be them facing challenges in their married life due to Oliver's vigilante duties. Maybe Felicity gets worried every time he goes out at night. Another could be them planning to start a family and all the joys and concerns that come with it. Or perhaps they take a trip together and encounter some old enemies during their journey.
Well, a plot could revolve around Felicity having a difficult pregnancy. Oliver then has to balance his duties as the Green Arrow and being a supportive partner. They could also have a story where they are trying to prepare for the baby's arrival while dealing with a new villain in Star City. This villain could be targeting pregnant women, making Felicity a prime target. So, Oliver has to stop the villain while also taking care of Felicity.
One idea could be that Oliver and Felicity's baby has some sort of unique meta - human ability that they have to protect it from those who would use it for evil. Another could be the baby getting kidnapped and Oliver and Felicity having to team up with their old Arrow - team friends to get it back. They could also face challenges in raising the baby while still being vigilantes.
Perhaps there's a villain who targets Felicity because of her pregnancy. This villain could see the baby as a way to get back at Oliver. So, Oliver has to go to great lengths to keep Felicity hidden and safe. Meanwhile, Felicity could be using her intelligence to find a way to outwit the villain from her hiding place, all while dealing with the physical and emotional changes of pregnancy.
Another plot could revolve around the challenges of raising a baby in their dangerous world. They have to constantly protect the baby from their enemies. Maybe there are villains who try to kidnap the baby to get to Oliver or Felicity. So they have to set up a super - secure hideout and enlist the help of their friends like Diggle.
A third plot idea could be a 'what if' scenario. What if they had their first time under the influence of a meta - human power that made them more open and vulnerable? This could lead to some interesting character exploration as they deal with the consequences of this unexpected event. They would have to navigate their feelings and the changes in their relationship while also dealing with the potential danger of the meta - human's power still being out there.
A possible plot is that Felicity discovers she's pregnant at a very inconvenient time, like right in the middle of a major battle plan against a big villain. This could lead to a lot of internal conflict for her and Oliver as they decide whether to continue with the plan or focus on the pregnancy. Also, they could face challenges in preparing for the baby, such as finding a safe place to live considering their dangerous enemies.
One interesting Oliver and Felicity baby fanfiction could be about their journey of preparing for the baby. It might include them decorating the nursery, choosing baby names, and dealing with the excitement and nervousness of becoming parents.
You can try some popular fan fiction websites like Archive of Our Own (AO3). It has a large collection of fan fictions about various topics, including Oliver and Felicity. Just use the search function on the site and type in 'Oliver and Felicity' to find relevant stories.
There could also be a storyline where they decide to start a family. This would bring in elements of how they would raise a child in a world full of danger and how their relationship would change and grow with the new addition to their family.