Okay so this idea is probably more suited as a standalone app than as a mod for phpBB, but here goes:
What I am thinking of is an event calendar, where events are basically topics in a special forum. The twist is that when you create the event, you invite people (members of the forum or usergroups), and basically it is a poll where people can respond if they are coming or not. It's kind of like evite (evite.com).
Now I figured this might not be too far fetched because there is already a calendar mod, and there is a mod that tracks the poll results including which users voted what.
I guess it would easiest at first to make it so everyone on the forum is by default "invited" to every event. But it would be nice if you could have events that are not advertised to everyone (like private parties or something), and maybe those can be communicated by PM or something.
Anyways, just wanted to put that out there to see if anyone has any thoughts.
I don't mind helping on the development. I know some php and have fiddled with phpBB a bit, but I don't think I have enough experience to tackle the whole thing on my own...