I just come across a crossover fic called "Lord and the Avatar" By Shadowsnake89 feat. The Legend of Korra and Thundercats (2011). It takes place after Book 4 and What lies above part 2. SUMMARY:When a portal opens bridging two Earths, the worlds' respective heroes meet face to face. Will they come to understand the others struggles and do what must be done to keep the Ancient Spirits of Evil from spreading? Maybe after they're done beating each other. Here's the link! https://m.fanfiction.net/s/11829449/1