The FCC is suggesting that spectrum licensees be allowed to let others use their spectrum in a similar fashion to the unlicensed bands: The only types of devices that would be allowed to operate in the "private commons" would be peer-to-peer devices in a non-hierarchical network that doesn't use the network of the licensee.
According to Kevin Warbach, a one-time counsel to the FCC and now an assistant professor at The Wharton School, some experts say that if open spectrum is truly beneficial, then licensees will create it. It seems there are a lot of doubters that licensees would actually open their spectrum for this kind of use, but perhaps a few innovative spectrum owners would give it a try and find success.
The FCC envisions that some current users of unlicensed spectrum would benefit by negotiating with spectrum owners to use their spectrum as a way to offer a service that is less crowded and thus potentially more valuable than services that operate in the unlicensed bands. It's the FCC's plan for avoiding the "tragedy of the commons," where the shared item becomes so overused that it loses its value.