The marketplace is primarily used to sell assets and development resources. There are a couple of showcases available, but as far as I can tell none of them use default engine assets. The advanced top down project, however, uses the default blue character.
That being said, you should continue to work on your project, and submit it when the time is right - Epic has the final say, and if you’ve got something awesome to show off it is unlikely that they wouldn’t be willing to work with you.