One common theme is adventure. Given the nature of the characters, they often embark on exciting adventures together in the fanfiction. Another theme could be the development of an unlikely friendship that gradually turns into something more. There's also the theme of survival, as both characters are often in tough situations in their original stories.
You can try some popular fanfiction websites like Archive of Our Own (AO3). It has a large collection of various fanfictions, and you might be able to find some great riddick x jack ones there. Another option could be FanFiction.net, though the content quality can vary more on this site.
Their relationship could develop further if they start to understand each other's motives better. For instance, Jack may start to see the reasons behind Riddick's tough exterior. This could lead to more trust between them.
Another plot could revolve around Riddick trying to teach Jack some of his survival skills. He shows her how to track in the dark, use minimal resources for maximum effect. This could lead to a deeper bond between them as they face various challenges together. For example, they get trapped in a cave system and have to rely on each other to get out.
Since 'lemon' usually refers to something not suitable, we should focus on proper Riddick fanfiction. In general, good Riddick fanfiction often features his survival skills. He is a tough character, so the stories might show him in dangerous situations and how he overcomes them.
There are fanfictions that focus on Riddick's relationships. Some might explore a romantic relationship between him and a new female character, perhaps a strong warrior from a different planet. This relationship could add a new layer of complexity to his character as he has to balance his solitary nature with his feelings for someone else. Also, there could be stories about his friendship with Johns, showing how their relationship might have developed differently if certain events in the movies had changed.
Survival is a common theme. Since the Riddick universe is so dangerous, the original characters often have to fight to stay alive. For example, they might be stranded on a hostile planet like Riddick himself.
Survival is a common theme. Since Riddick is often in harsh environments in his own story, when crossed over into other universes, he still has to fight to stay alive. For example, if crossed with 'The Hunger Games' universe, he'd have to use his skills to outwit the other tributes and survive the deadly games.