I am not into composing (yet, but I intend to start

) but I am an experienced writer.
You know, it is said that there are only 7 original plots out there, and all stories ever written are just derived from these. Sometimes you can boil it down even further: every story starts with "a stranger comes to town" or "someone goes out on a journey". Or
even shorter: a state of balance is disordered ...
So, this means you should not bother trying to make up a "original" plot at all. Just rely on yourself, because you are unique and your story will be too, in the way you tell it - if you don't try to copy someone else, that is. Npw, let's paraphrase this to music composing ...
No, I cannot "make up a story plot" just out of thin air. You have to give me something, a starting point. A little trick is to copy a plot from another genre, or even a news notice, and move it to another setting. Take an event from today's headlines and paraphrase it to an SF or fanatsy story. Take a very old story, an ancient Greek drama for example, and make it modern. Let a story about a conflict between two countries be a quarrel between two neighbours in the suburb. Or the opposite ...
Or take a photo by random and tell a story about it, see where your imagination will lead you. Or take three words by random and make up sentences with them. Anything will do!
So how do you do this in music? Well, listen to a genre you normally dislike. Say you don't like country music. Steal a few bars from Kenny Rogers or whoever and strip it down to the pure melody, then rewrite in Beethoven style. Or the opposite. (It's done all the time, by the way ...)
Or use a photo, look at it and describe it on your piano. Or do as a rock star I know, who creates melody lines out of interesting verbal expressions, and sometimes does the opposite. Steal an old poem.
I think what you need is to play around, mix the genres, experiment with old ideas. Most of all, don't be afraid of making something "bad". If I demanded from myself that every writing attempt would be great, I would never achieve anything. My good ideas are always developed from bad or lame ideas!