+1 for integrating with Pro Sites.
I am creating a network with users that fall into very different categories but do have logical interactions.
A subdomain on the primary domain would lead a visitor to believe that the site is for a doctor. Yet I will have companies that sell to doctors, schools, continuing education providers, etc... All in the same network. So in a membership scenario it would be great if a package could include a site or sites only on a specific domain. Example maindoctorsite.com only for doctors, doctorproducts.com for companies that sell to doctors, doctorschools.com for schools...