I know this is a big change, but have a read. There may be something you find interesting here.
Why can there only be one menu (Activities)? Wouldn't finding your target be faster with more? Also, I think GNOME should stop working in an application based way. I don't generally care what I'm using to view my video, I just want to see it. My concept totally changes the way you use the computer.
First image: Documents The first one is similar to current and activities menu. The menu here uses the Zeitgeist framework. You can search quickly if you know exactly what you're after with the bar at the top. If you don't and it isn't something in the most recent documents, click older documents, leading to image 2
This shows more docs, going back further. To browse through time, click the left and right buttons on the ends, which slides the whole lot left 1 column. If you go left past the most recent documents, you get options for new docs. You can also filter by Name, type or author.
Clicking a document opens it. Or you can right click to get other actions. (Alternative idea - clicking opens menu offering actions. E.g. for a video: watch video, edit video, send video to person, send video to device, delete etc)
The other overlay at the moment is people. This could be powered by soylent.
There are two initial menus - online and favourites. These both get filtered by the types at the top. You can view more Online or Favourites by clicking the respective more button. This opens a bigger menu for each. Anyone with unread communications to you is marked and gets brought to the front. People have the relationship and their status (including location)
Clicking a person brings up an actions menu (next image) . The contents of this is dependent on their status and what programs/services they use. If they are online and use e.g. jabber, you can IM them etc.
There could also be other overlays. A status one which could contain widgets e.g. weather forecast, news feed. A system one with config options and log on/off. For compatibility, an apps one.