There is a theory that Hangul may have been inspired by, or based on, the Yuan era Phags Pa script. How accurate is this claim, and are there any historical documents attesting this?
This theory is based off of an excerpt from the Annals of Sejong (Dec. 30th of the 25th Year & Sept. 29th of the 28th Year) which says Hangeul was "moulded from an older script." Phags Pa is just one of the writing systems that has been suggested. Obviously, it will have to remain a speculation.