Similar to something like JMS, or other messaging middleware interfaces (MQ Websphere, Tibco, etc), but PHP based obviously, I'd like to see forum software work as it should, but also publish events which can be captured locally, over a network, the internet, etc.
This might make third party integration easier, and allow more flexibily in developing xenforo add- ons.
In its simplest form, if a person registers or creates a new thread, an event message is broadcast to notify a client subscriber of this message that the event occurred, along with the details such as some limited user information from the database. A client app listening to "new registration" messages wouldn't have to query the vbulletin user database to find out if the user has registered or not.
This of course, would rely on encryption of the message, and rules/protocols put in place for client apps to subscribe to, and interpret these messages. It'd also be difficult in deciding how to re-publish messages that are lost, how to determine if a message was lost, priority lists, etc.
A system like this would also keep external third party apps away from the vbulletin database and code, and could extend xenforo into a messaging service center, serving as a driver for other applications.
Well, like I said, it'd probably be a difficult task to come up with now that I think about it some more.