I don’t have a rubric to share, but be sure that the score is based upon what is the point of the writing assignment. For example, if writing a dialogue, it should sound like a real dialogue when it is read.
A book to consider is If You’re Trying to Teach Kids How to Write…You’ve Gotta Have This Book.
It’s full of a variety of writing activities, from making lists to writing letters to imaginary journal entries or dialogues.
Consider having your students write dialogues, character sketches, beginnings of stories, chapter titles for an imaginary book, imaginary interviews with historical characters, captions to pictures, responses to famous paintings, alternate endings to stories.