You can start by looking on Archive of Our Own (AO3). It has a large collection of fanfictions, and you can easily search for 'Jack Harkness x Doctor' there. Another place could be Fanfiction.net, although the interface might not be as user - friendly as AO3. There are also some Doctor Who - specific fanfiction forums where you might find some great works about Jack and the Doctor.
Well, 'Journey with Jack and the Doctor' is a fanfiction that stands out. It has an engaging plot that keeps you hooked from start to finish. Then there's 'Jack and the Doctor: Parallel Paths' which is interesting because it shows how their paths cross in unexpected ways. Also, 'The Meeting of Two Legends' is a great read as it focuses on the first encounter between Jack and the Doctor and how it sets the tone for their future relationship.
Romance is a very common theme. Writers often explore the relationship between Jack and the reader character, from the initial attraction to a deeper emotional connection. Another theme is adventure, as Jack is known for his time - traveling and exciting exploits, and the reader usually gets involved in these adventures too. There's also the theme of mystery, given Jack's own mysterious past and nature.
One great 'jack harkness x reader fanfic' is 'A Journey with Jack' on FanFiction.net. It has a really engaging plot where the reader character gets involved in Jack's adventures across time and space. The writer does a wonderful job of capturing Jack's charm and charisma, making it a very immersive read.
It's possible that their encounter would be a more serious and emotional one. Jack has seen a great deal of loss and the 13th Doctor also has her own emotional baggage. They could bond over shared experiences, like the loss of friends or the struggle to protect the universe. Jack might offer some unique perspectives based on his long life and the Doctor could in turn show him new ways of looking at things.
Perhaps they find themselves in a situation where they have to rescue a group of people trapped in a parallel universe. Jack's skills as a former Time Agent and the Doctor's ability to navigate difficult situations would be crucial in this rescue mission. It could also be a chance for them to learn more about each other's pasts and grow as characters.
Well, if you like a bit of mystery in your fanfiction, look for ones where the reader has some sort of hidden power or past that Jack is intrigued by. For example, there's a fanfic where the reader is from a long - lost alien race and Jack is trying to figure out what that means for the universe and for their relationship. These types of stories add an extra layer of depth to the Captain Jack Harkness x Reader dynamic.
Archive of Our Own (AO3) is a great place to start. It has a large collection of fanfics in general, and there are many 'jack harkness x reader' ones. You can use the search function and filter by tags to find exactly what you're looking for.
A Jack Skellington crossover with Corpse Bride would be really cool. Since both are from the realm of the spooky and undead. They could team up to solve some problem in the underworld, like a new threat that endangers both their after - lives. Or perhaps a crossover with Disney's Moana. Jack, being curious about other worlds, sails to Moana's island. He tries to understand their way of life and the magic of the ocean, and in the process, he teaches the islanders about his world of Halloween.