A channel has a relatively short description such as "PHP", "MySQL", "sci-fi", "barefoot", maybe analogous to Twitter hashtags. In appearance a channel node looks similar to a forum node. An XF board can then create a channel node which means it subscribes to that channel, which again means it will synchronize the threads within it with other boards that are subscribed to the same channel (probably using some kind of distributed web service).
In the configuration it should be possible to set the retention time for the threads in the channel node, since presumably there could be a lot of threads, and not all of them might interest the local users. There should be a separate setting for the retention time of threads where local users have posted. One of the options should be "forever". It should also be possible to configure how often and when the synchronization job should run. I envision this as a distributed network with no central node, so I guess each board has to be able to both "receive" and "broadcast".
(This is a slightly more distilled and concrete idea than my previous thread about a "Xenforo Network".)
Thank you for your time reading this. Now, let the nay-saying begin
Edited to add:
It might be a good idea if only threads where local users had posted got indexed by search engines. And it might make sense if users could choose whether or not they wanted to see each of the channels.