That gives us liberty to mine anything for plot. After all, Shakespeare did.
Nab off with anything from anywhere: fairytales, films, ballads . . .
1. Prophecy: Play Mother Shipton for ten minutes. Pick any character you like. Use the second person, future tense to foretell what will happen to them. Captain Hook, you will be born into a seafaring family but turn to piracy . . .
2. True Confessions: Your character has only ten minutes left to live. They must get their last secrets recorded for posterity.
3. Fortunately/Unfortunately: Try this format to begin with. There once was a ______ who ____________. ( character & setting /premise).
Write alternate sentences beginning with Fortunately or Unfortunately. e.g.
There once was a gorgon who loved the snakes in her beautiful hair. Fortunately, they were not poisonous. Unfortunately, she bragged about their hunting skills which annoyed the goddess Artemis.
4. Webs: You'll need plain paper for this, ideally. Maybe coloured pens if you like.
a. Write the names of the key characters in a story of any sort in random places well-spaced out on the paper. Probably best to put the major protagonist(s) in the middle.
b. Join them up with lines of cause and effect in note form.
So in Beauty and the Beast, Belle would be joined to her Father because her request for a Rose led to him stealing one from the Beast's garden. The Sisters would be joined to the Beast and Belle because they made Beauty stay longer at home than she promised, which made him ill.
Version 1 If at all possible, find another writer to do this with. Online is fine!
Write the first sentence of a synopsis ( of any story, familiar or not). Your friend writes the next and so on.
If you can't do this, try Version 2 ( you will need to cut and paste literally or figuratively).
Recount a story for four minutes in simple sentences. Using no obvious names helps.
i.e. A man is washed up on a desert island. He is all alone so he builds a camp. He catches animals to survive. One day, there are footprints.
Then recount a different one in the same way. For the last two minutes, interweave them to create something new - you can get rid of bits, put two sentences of one followed by three of the other, place them in any order - whatever works!
All of the above exercises can be done with your own work if you ever get stuck. There is nothing like some silliness to get the creativity going again. The Muse likes a bit of fun.