Those are all good possibilities [for a novella Brandon is working on], but it could also be the one he mentioned during the first Book 4 update where an apprentice of Hoid is stranded on a minor shard world with an umbrella-based (Edit: oops!) magic system.
Please. It was a KITE based magic system. An umbrella-based magic would just be silly, now, wouldn't it.
I'm tempted to plot that one as a YA book, to be done after the Skyward novels. I'd like to do something YA in the cosmere, but have been waiting for the right moment.
Would Mistborn Era 1 not count as YA? I got those vibes from many parts of it, though most/all traces of that vanished in Era 2.
Mistborn Era One walks the line. (Having a teen protagonist for one of the viewpoints does indeed give it the vibe--and we did a YA printing of it, with YA covers.) Really only the first book would fit, though, and it's kind of a stretch.
Generally, YA is defined by the age of the protagonist, and the conflicts they're seeking. Vin's quest to find a home where she can trust the people around her is a YA style plot, but by the second book, it's drifted more toward adult style plots.
A lot of sf/f walks this line, though. There are arguments for some Game of Thrones to be YA for similar reasons. (Be aware that in the industry, content--meaning sex/violence/language--is not a descriptor of whether something is YA or not. It's age of the protagonists and the nature of the conflicts.)