The way I figure it, it would go down something like this: Shakespeare, established playwright, needs a new patron after Elizabeth's death, and naturally is looking to please the new King James of Scotland. So in his research, preparatory to writing a play for said Scottish King, he discovers the AWFUL TRUTH. When the Templars fled the Inquisition in the 1300s, they came -- as everyone knows -- to Scotland. But they brought with them a secret treasure, looted from the hidden tombs of the Pharoahs of Egypt. And these were ancient mummies, who rose to new life in northern Britain and spread their dark power through fell and unbreakable curses. And James is their pawn, their slave, as he would give anything to earn the gift of immortality, which will be bestowed upon him by his master, the Mummy named MacBeth.
"MacBeth with Mummies" then turns into a "runner" plot, with Will trying to stay ahead of every Mummy from fiction or legend I can find, all of whom are closing in on him as he writes his expose of the entire magical plot, a play named "MacBeth." But, in the end, the best he is able to do is barter for his own life. He will be allowed to live, and James will indeed become his patron, securing both Will and his family's future for a generation to come, but his play is to be destroyed. The truth cannot be allowed to be revealed. But, since Will has already told Henslowe and others that a "MacBeth" play is coming, one will be written for him and put on its place, one which is more flattering to James, and which does not reveal the hidden truth, and which is safe and harmless. And this play, the fake MacBeth, has already been written, by James's Master, the Mummy named MacBeth.
It's sort of like a 17th century Anno Dracula.