Basically, yes, the document object is designed to be extended by user code, so you could add your own kinds of nodes (even allowing them to contain existing no

Basically, yes, the document object is designed to be extended by user code, so you could add your own kinds of nodes (even allowing them to contain existing node types, as in a table), but for some nodes (like tables) the impact will probably be big enough to require some specific UI-related code in the core.