I really think anything Weird Wars related would be a great opened ended campaign. Weird Wars has always had a special place in my gaming heart.
It all started as a kid reading the awesome Weird War Tale Comic Books!
It then went to Weird War GURPS, which was a total riot to run!
Which then morphed into the Savage Worlds Weird War Series..Which are all awesome, Tour of Darkness, ROME, Weird Wars Two, but Weird War One takes the cake... so much info and imagery to pull from!