One can, and many people have, over the years. Some of the more interesting ideas that people have had, in my view, have been: * ID systems that introduce hie

One can, and many people have, over the years. Some of the more interesting ideas that people have had, in my view, have been: * ID systems that introduce hierarchies, allowing (say) a user to create multiple sub-users (one for running the WWW browser, one for running the office suite, one for running the chat program, ...); * proper nonce ID creation with segregation guarantees (c.f. nonce SIDs in the Windows NT world); and * IDs that are reference counted, accessible via descriptors, passable from process to process via descriptor-passing mechanisms, and explicitly supplied in system calls for opening/creating things.