Since no one has gotten it right, here’s a compilation of all the guesses so far to give you all a hand:

1. A kid finding a cursed Dean
   
2. A kid finding both cursed brothers (not the mystery AU, but a possibility for BL)
3. Field Borrowers
4. Both brothers are cursed and John saves them
5. Reversed Brothers Together, where Dean got cursed and saved by Sammy
6. Hitchhiking Sam, off to find his brother
7. The witch gets Sam when he’s first cursed
8. Sam/Dean grow to be a giant instead
9. A brother is cursed and either Bobby or Gordon finds them
10. Sam is cursed as an adult and meets the Watch family later on in life instead of as a child
11. The trickster switches the brothers sizes (likely for a story within an AU, not an AU by itself)
12. Sam adopted by a human family after being cursed
13. Sam caught by other humans with no one around to save him
14. Sam is cursed at a different motel
15. Sam is adopted by sprites
16. Sam and Dean are borrowers for most of their life
17. The Winchesters genderswap
18. Sam raises a family of his own that fits in Dean’s pocket (just plain adorable, really)
19. Sam and Dean shrunk as adults (more likely for a canon story, since the curse in BA only works on children)
20. Borrowers and humans coexist
21. Sam had to survive on his own
22. Dean discovers Sam earlier/later on in life
23. Both brothers cursed; one has the curse broken

All good guesses but none hit the mark! If anyone gets it right, there will be a sneak peek put up for everyone to see!

Other points of interest brought up:

• The AU will involve one or both of the brothers being cursed
• The curse will happen at Trails West
• Bowman or Oscar may appear in the AU, but not as one of the central characters
• Celeste is involved to her normal extent; as the source of the curse
• One of the three campers from Fairy Tales: Bowman of Wellwood is involved in the AU
• The guesses that come closer to the mark without hitting it have been bolded!