I've wanted to run a time travel campaign on an alternate earth setting. Allowing the characters to interact with historical events seems like a cool way to learn about them.
I have this vision in my head of fantasy characters in a trench warfare scene with Fireballs and Cloudkills going off and swarms of crossbow bolts erupting from a machine gun nest.
The challenge for me seems to be finding a continuity thread to link the time frames and paring down to just a few to show to the players. My current idea is like the old TV show "Voyagers" with characters falling into a time frame to "fix" something (different mechanics than "Quantum Leap" but similar to that too). WWI at Verdun, Mecca just as Islam is starting, the voyage of Christopher Columbus, and the coronation of Charlemagne are the major scenes. I'm still debating these and looking for more dramatic choices.
Anyway thats my two cents on a time travel campaign
Stoner