Unusually forward-looking, as this dinosaur of the net has been slow even to consider a few extra TLDs, never mind opening it up completely.
No technical reason why it can't be done, a couple of the alternate roots, including one that is still up and running -
UnifiedRoot - Top Level Domain Registration - UnifiedRoot - had "unlimited" roots.
Thing is, alternate roots such as that one, or New.net, can generally be ignored, as presence in the main root that everyone uses is more important, the alt roots are merely "vanity domains" as the majority of people will not be using them.
If the "proper" DNS root does start using them, then it cannot be ignored, but it seems there will inevitably be a "gold rush", as owning a TLD would potentially give a company a one word web address.
