I'd love to see the new plugin or an add on to 'User Synchronization', which lets users choose the site they want to join. So if you do have multiple wordpress / buddypress sites your members can see all your networks, and they can choose to sign up for them too just by clicking one button. AMAZING!
Maybe even if the wordpress / buddypress sites are not run by the same people, they could add the plugin to each others sites to help promote each others wordpress / buddypress networks.
If you all get behind this concept the nice people at wpmu dev will look into this.