Personally, I think some of Shakespeare’s plays were group efforts.
You may be right some say he stold a lot of other peoples works.
Many authors get ideas from other peoples work. I would not be surprised if Bill took promising plot lines from others and made them blossom. I also would not be surprised if he did research on the mannerisms and attitudes of the nobility by talking to their servants and retainers.
Take Romeo and Juliet. You can see traces of this from Tristan and Isolde and even back to the Puranas and other Hindu-Aryanic tales
Ditto for King Lear
however, Shakespeare polished these and made them incredible. Why Shakespeare is read not only in english but in a multitude of languages -- I've read him in French and in Polish and my sis-in-law in Japanese and the themes and plots STILL hold
That is mastery.
Sticking to the Japanese point -- have you see Karusawa's "throne of blood"? it's based on King Lear yet stands on its own.