Lately I've been tumbling a few Cosmere fanfic ideas around in my head (spoilers unmarked).
A Mistborn fan gets transmigrated into Era 1 a couple years before the fall of the Final Empire and has to deal with all the chaos that involves. The main draw in my mind is that you have all this meta-knowledge about the politics and magic system and whatnot, but you have to be very careful about how you apply it in order to 1) avoid tipping of Ruin that you're onto him and getting a dozen Inquisitors pointed directly at you, and 2) avoid messing up Preservation's plan through the whole trilogy and accidentally dooming the world. My original thought was to have the protagonist get dropped into one of Straff's allomantic bastard children when they snapped (probably a soother), but you could also go the 'Villainess Reincarnation' route and drop them in Shan Elariel, which comes with its own bag of problems.
The Lord Ruler dies, and instead of just stretching off into the Beyond he gets time loop reincarnated as a skaa child a couple years younger than Vin (or, as above, winds up in their body after they're beaten to the point of snapping). Gets to deal firsthand with all the consequences of the dystopia he created without the godking powers that let him justify himself in the original timeline. (This is probably my favorite of the three, at the moment).
This one's a little different and is inspired by another fic called Life Ore Death by Obloquy, which is a crossover about a young feruchemist terriswoman from Era 1 winding up in the Young Justice/DC Comics universe. Good read, I recommend it. Anyway, this fic takes the same basic premise but instead the crossover character is an Elantrian and her seon who are from about 3 years after the events of the book. Of course, Elantrian magic is location-restricted even without multiversal travel, so her main toolkit (at first, anyway) is peak-human strength/durability and low-level healing from being so heavily Invested, her wits, and having a flying ball of light as a sidekick.
