If this plugin/ability already exists, then I appologize.
It would be great to be able to let my users rate themes that are available to them within WMPU, being able to rate on 1-5 stars, just like on the wordpress.org themes directory.
Then, when choosing a theme for their blog, let them be able to sort the themes by Popularity (how many blogs within the system are using each theme) or Rating (sort by most popular).
Even giving users the ability to add themes to a “Favorites” list, so that they can simply add 5-10 themes to their favorites and quickly find them for their blogs.
When there are more than 100 themes available for users to pick from, it can be daunting to try to find the right one, even to find a specific one if you know it by name, since the only way themes are listed currently is alphabetically.