So I’ve been on a serious 'How to Train Your Dragon' and 'Rise of the Guardians' crossover kick lately. The forums and dedicated fansites are honestly where I find the most developed Hiccup x Jack fics. The fanfiction community on Tumblr, especially with the 'rotg x httyd' tags, is super active and curates really well. Users will reblog with extensive tags like ‘found family’ or ‘frosthawk’, making it easy to sort through the mountains of content. I’ve also had better luck on Dreamwidth communities than on some of the bigger, more general platforms—the discussions there tend to be more focused on character analysis, which leads to stories that really nail their dynamic.
AO3 is the big one, obviously, but you have to know how to filter. Just searching 'Hiccup/Jack Frost' will get you a mix. I always filter by kudos and then check the bookmarks of authors I like. Some writers who excel at this pairing don’t always get the most immediate attention because crossover fandoms can be smaller. I found one of my absolute favorites, a slow-burn about them as spirit rulers dealing with mortal memory, by sorting by ‘complete works only’ and ‘word count >50k’. FF.net is a bit of a graveyard for it now; the good stuff is often older and buried under less polished work.
The real magic for this specific ship, I think, is in the thematic overlap. Both characters are kinda isolated innovators connected to elemental forces, so the best platforms are ones where writers dig into that. Spaces that encourage meta-commentary, like specific Discord servers, often have channels where people share their WIPs or recommend hidden gems you’d never find with a simple search. My personal bookmark list is mostly from a server that’s been around since 2015, so those long-term fans have collected the absolute cream of the crop. Those smaller, tight-knit communities consistently have a higher quality bar for this pairing than the front page of any major site.