Just of the top of my head:
Around the world route maps (people submit the trips they did)
Greatest explorers route maps (although I guess there might be some problems with historical accuracy)
Geek Spots Around the world
Known Military Bases around the world (and perhaps a strategy game on top)
The use of large scale maps are IMHO still a very underutilized area because most go for the location based stuff.
Also though in tracking that down, I found this enormous (2300+!) catalog of Google Maps mashups:
http://www.programmableweb.com/api/google-maps/mashups
Just of the top of my head:
Around the world route maps (people submit the trips they did)
Greatest explorers route maps (although I guess there might be some problems with historical accuracy)
Geek Spots Around the world
Known Military Bases around the world (and perhaps a strategy game on top)
The use of large scale maps are IMHO still a very underutilized area because most go for the location based stuff.