Michonne and Rick aren't together in the comics. The focus of their characters and the overall plot doesn't bring them together romantically. However, their individual arcs are still significant and contribute to the depth of the story.
One great fanfiction could be 'Surviving Together'. It shows Rick and Michonne in a post - apocalyptic scenario where they rely on each other more than ever. Their relationship development is beautifully written, from hesitant allies to deeply in love partners.
The significance could be that it changes the entire flow of the story. Their relationship was a major part of the original narrative, and an early meeting would rewrite a lot of the plot. It might mean that they are able to save more people or face challenges in a different way.