Currently drupal.org doesn't have themes/designers competition and just to be clear to all: Example:
DotNetNuke CMS has the "Design Challenge Competition" so vote this idea so it would be add on e.g: drupal.org/competitions/Theme
But to be clear, this competition must have rules in it and those rules are going to take place if this idea got the highest votes and this will improve drupal.org entirely =).
And yes same for Modules too,
the Challenges can be listed for developing a module and so the community can submit the Dev-modules related to the Challenge and win.
Win a Badge or Even Cash prizes too, well that would also depend on project itself.