You have three options:
1) create a theme for independent WordPress installs. This is the easiest route in that it is the least restrictive. You can find out more about this at WordPress.ORG
2) contact the theme creators on staff at Automattic here and ask if they're interested in working with you on creating a theme specifically for WordPress.COM
3) just tinker with CSS skins for existing themes here and make them available for people with the CSS upgrade. You don't need anybody's permission to start doing this, and there are lots of examples at http://csswiz.wordpress.com and similar sites, but editing the CSS of themes here is not like starting from scratch, so be prepared for some limitations.