Before diving in, writers should ask themselves: What should I write? What do I want to write?
The answers might not be the same.
The “should” response may include matters of skill and marketability; e.g. you feel competent to write a particular story and/or there’s a market for it.
I should work on polishing the rough draft of a SF novel for young adults that I finished and put it aside to “simmer” before the rewrite. YA novels sell well now, and the premise of mine generated interest from an independent editor.
The “want” response is an emotional one. Characters appeal to you. Writing the action scenes is fun. The conflicts get your heart pumping.
What I want to work on is a fantasy novel for young readers who are advanced enough to handle higher vocabulary but not ready for the content of YA novels. I started working on a series of stories about these characters when I was child.* The setting feels like a second home and, with little preparation, I can pick up where I left off in the story.
Weigh the options and make a choice.
For me, the fantasy novel is the right choice. The SF novel requires research, including a trip to a currently-closed library.
* I remember tossing it into a burning barrel when I left home for the last time. However, my parents later delivered a box of source material they found in a closet: written scraps, typed character studies, and notebooks!