Do you have some particular options in mind? Practically any example I am can come up with ends up devolving to a single namespace.
The least restrictive one I can recall is the Usenet namespace where the equivalent of a subdomain didn't — at least in principle — require permission from the domain "owner" to establish (eg. alt.comics.superman.dies.dies.dies). In practice this required coordination and consensus among various administrators for a new group to be established, and under various scenarios existing "owners" could object.
A more practical take, I'd love a domain name system that only promised that, at some time, i was the owner of a certain DNS domain. If I stopped being the owner, I could still continue hosting my site, my identity would be cryptographically well known & certifiable, people would know who to come to (or maybe not, not a hard requirement for this scenario; like HIP they should know me when we talk & i do want to say it's me), even though the link, the "ownership" (renter-ship) of the domain i'd held had expired. This again is another huge host of interesting messy hard problems, and if someone does get my keys, i'm hosed, but i really like how it favors a very strong distributed identity over this pressing incessant badgering need DNS has to enforce consistency & singularness.