Nothing is more paralyzing than the idea of limitless possibilities.
The best advice is not to write what you know, it's to write what you like.
Write the book you want to read
Every new idea is just a mashup or a remix of one or more previous ideas.
The thing is: It takes a lot of energy to be creative. You don't have that energy if you waste it on other stuff.
You're ready. Start making stuff.