Although Game of Thrones overtook George R.R. Martin's books long before it ended, the Jon Snow sequel show could still use one A Song of Ice and Fire character the HBO series didn't use. A Jon Snow sequel is reportedly in development at HBO, which would see Kit Harington reprise the role. It's a surprising move, given Game of Thrones' ending wrapped-up Jon Snow's story rather neatly, and the focus on spinoffs has been prequels set long before the timeline of Jon, Daenerys Targaryen, et al. Despite that, however, there are some intriguing directions it could go in.
Game of Thrones mostly passed Martin's books in season 6 (though even before that it was making increased changes and cuts), which inevitably included alterations to Jon Snow's story. The biggest element - Jon Snow's death at the hands of mutinous Night's Watch brothers - was kept intact (albeit again with some diversions), but otherwise there were broader changes to that plot. That included cutting many aspects of the Wildlings themselves and their part in the story, with one twist seeing Mance Rayder survive, his body switched with that of Rattleshirt who dies in his place, and infiltrating Winterfell, missing completely.
Another story and character Game of Thrones cut from the books was Val, a Wildling "princess" - the sister of Mance's wife, who also didn't make it to the adaptation - who becomes a key figure in Jon's arc. So far, it's unclear exactly where Val will end up, which made it easier for Game of Thrones to cut her; the show's version of Jon did work, but would've been even stronger with Val there. She represents a temptation to him that he must resist, an opportunity to have the family and life he dreamed of. But she is also a strong tether back to the Wildlings, to the lands where he first learned to love and, with her, could do so again. She is a chance of a happy ending that he often refused to allow himself, and that could've made his fate all the more impactful. Still, though it can't directly adapt the same story, Game of Thrones' Jon Snow sequel could easily use the character of Val, since the Wildlings would have to play a major role in it.
How Val Can Appear In Game Of Thrones' Jon Snow Sequel
Game of Thrones may have cut the book story, but there are a couple of options for how the Jon Snow sequel could include Val. Jon and Tormund Giantsbane went North with the remaining band of Wildlings, most of whom are unknown to audiences. If the sequel is to focus on this setting, then it'd need to flesh some of them out anyway; introducing Val as someone among their ranks is an easy way to do it. It'd be a little surprising that the Mance Rayder link hadn't been mentioned before, but it's not impossible to use it, and otherwise most of her core traits would remain intact. She could still be a fierce, courageous, beautiful, and skilled Wildling who can both work as a character on her own terms and through her chemistry with Jon Snow. She proves herself as capable as anyone in knowing and ranging in the lands beyond the Wall, which could be used depending on what conflicts arise and threats emerge in the Jon Snow spinoff.
Alternatively, Val could be positioned as more of an antagonistic force. The question of who Game of Thrones' Jon Snow sequel show's villain will be is a pertinent one, and conflict between different Wildlings is a possibility. Val is someone who would be respected and could lead others, and perhaps even challenge Jon's own authority, again with or without the connection to Mance (though that could help her claim). A battle to unite the Wildlings, with Jon as an outsider (to some) once again, could form part of the spinoff's storyline, and Val would be a worthy opponent for him in a way that could still keep aspects of their book storyline, again allowing for hints of romance and temptation. Val is one of the best characters in Jon Snow's books story, and someone who would strengthen his connection to the Wildlings and provide a great foil for him whichever way the show adapts her.