That's not how it's done.
The producer plots the season, which character they want the focus of each story and maybe three lines as a general outlline idea seed, and then they make the writers room fight over who gets what.
The producer then says to the room "I need 3 Chakotay stories, one of them has to be about his family, the other two make him look like an sshole because Beltran has been pissing me off, 2 B'Elanna stories possibly involving tribbles, and a Harry story about anything, can I see a show of hands? Who wants what?"
For some reason I think Berman stopped accepting Spec Scripts at some point, but I can't back up that cruddy memory of something someone here said.
(The room was mostly virtual constructed with lots of expensive long distance phone calls.)