First Function - Override theme of website depending on domain name.
Second Function - Assign nodes to be either shown or hidden from each domain.
Third Function - Allow domains to require approval before seeing the some or all of the nodes of that domain. (a simple automated request to be added to a permission group)
I'm not really good at describing exactly how I want it all to work so I made a diagram that I think, and hope, explains it exactly.
Is this a possible plugin? I think it would be very useful to many members of the community!
The reason I said not multisite in the title is because multisite is not compatible with several plugins. I hope a plugin with just these few functions should not be conflicting with the plugins already installed on Xenforo... or so I hope.