Alright, starting in about two to three weeks i'm going to have a hell of a lot of time on my hands. So, i've decided that i'm going to take on the task of coding a major feature for wpmu. Below are a few ideas that I came up with but if you have your own please tell us about it. Basicaly i'm going to leave it up to you guys to decide which feature you'd like best.
1) Calendar System - This could be done either by allowing a user to import data through a service like Google Calendar or a custom calendar script could be written just for WPMU.
2)Photos - I'm up in the air on this one because I can't decide whether it's worth it to build from scratch a photo management script when there are services like flickr that are extremely cheap. This feature would also require you to give your users more space and let's face it, unless your wp.com most people can't aford this.
3)Messaging system - Basically it would be a giant PM (Private message) system for wpmu. There would also be an xmlrpc type setup where users would be able to send PMs to other WPMU sites running the plugin. Basically userA at bunnyblogs.com could send a message to userB at rabbitblogs.com. To make it simple, the address for cross system messages would be similar to email "userb@rabbitblogs.com". Think of this as "Blog Mail" if you will.
Anyway, those are just a few ideas off the top of my head. If anyone has another idea feel free to tell us about it. :)
To keep from having an endless debate on this i'm going to set a deadline of May 13th (Saturday) for any new ideas, then i'll setup a simple poll the next week for you guys to vote.