There might be a fanfiction where they time travel to the future. In this story, they could see the consequences of their current actions and have to figure out how to change things in the present to avoid a dark future for Star City. They might encounter future versions of themselves or their friends, which would add an interesting dynamic to the story.
A great Oliver and Felicity time travel fanfiction could be about them getting accidentally sent to a different timeline. In this new timeline, things are very different. For example, Oliver is not the Green Arrow and Felicity has a completely different role. They have to find a way to get back to their own timeline while also trying to fix the problems in this new one. They might face challenges like not being recognized by their friends in this new timeline or having to deal with new villains they've never encountered before.