Now you have a great list of idea, you need to pick the one you think could grow into a story. Usually it’s the one you can’t stop thinking about.
Don’t try and analyse if people will like it, if you want to write that type of story, if it is commercially viable. None of these reasons qualify at this stage. For now, you just need to pick something fun that you would enjoy expanding on.
I’m going to pick my last idea from my list: the mother changing into an Alien. Why? Because there’s a big extreme: mother to alien. Not a woman to alien; a mother. A mother means she has a child, which creates more questions: is the child an alien hybrid too?
Whatever story you’ve picked spend some time writing it out. Don’t care where the story goes, or if it’s too complicated for your skills, just let the ideas tumble out of you and capture everything.
Here’s my expanded paragraph with some details in brackets:
A mother (with child) who discovers (after a car accident) her genes are mutating (stretching, morphing, changing texture whilst in the hospital) into an alien. She is being hunted by a mutant race who want her child and the Doctors know something she doesn’t.
Now this is developing well. I’ve got a few locations, a child they love (hopefully), a car accident (could make a good start to the story), a hospital (people who she ‘should’ trust, and look after her best interests, maybe), and clear opposing enemy: a mutant race after her child!
This begs the question: Why are the mutants after her?
Stretch out your idea. Don’t compare it to mine. Just brain dump everything. Maybe tighten up the details if you’ve got some good ones you like. Think about extremes: who is good, who is bad, where is home, where is danger? As long as your idea is still weird, cool, exciting to you, it’ll be weird, cool, and exciting to the reader.