These would be difficult to aggregate because unique visitors are site specific. Maybe a weighted sum could be used? Relative proportions of:
This would indeed be a great feature.
I would also include number of visits, search engine market share, all the reports under Visitors > Settings.
We could also have the Piwik administrators specify a Category for their website which would allow to give breakdown of these metrics by website category.
The reports could be available on the piwik.org website in a "Internet Report" page that would lists the number of opted in websites being tracked, total traffic, and the various available reports. I think we should build this as a plugin installed on piwik.org/demo that would receive all the opted in metrics and build the aggregated reports.
If anybody is keen to build this, please shout :) This sounds like a very interesting data intensive project.
Data we could also share