Yes, there are quite a few. You can start by looking on fanfiction websites like Archive of Our Own. There you'll find various takes on their relationship, from angsty ones exploring their rivalry to more creative interpretations that might show them in an alternate universe where they could be friends or something more.
The contrast between their characters makes it interesting. Ivypool is often portrayed as a complex character with her own internal struggles, and Hawkfrost is a rather mysterious and somewhat devious character. Their relationship in fanfic can explore how these different personalities interact and potentially fall in love.
Fanfiction.net could also be a source. But be aware that the site has a large volume of content, so it might take some time to sift through. Look for the Warrior Cats section and then specifically search for stories involving Ivypool and Hawkfrost's relationship. It's important to note that while exploring these platforms, respect the authors' works and the rules of the respective sites.
One challenge could be their different loyalties. Ivypool is loyal to her clan, while Hawkfrost has shown time and again that he is more self - serving. This would create constant conflict as they try to be together. Another challenge is the distrust from Ivypool's clanmates. If they knew she was involved with Hawkfrost, they would be suspicious of her motives.
One possible plot could be that Ivypool, while exploring the forest, stumbles upon a vision of Hawkfrost. He tries to manipulate her into believing that she can gain great power if she follows his teachings. Ivypool is initially intrigued but then realizes his true motives are to cause chaos in the clans. She then has to find a way to resist his influence and protect her clan from any potential harm he might cause through her.
Well, the differences could range from how the plot progresses to the way characters are depicted. Sometimes the anime might add or omit certain details for better visual storytelling, while the manga might focus more on inner thoughts and background information.