Re: Time Travel in Sci Fi and Games....The Good, The Bad, and the Oh so Ugly...
Another angle to consider is the various methods of placing characters in different times. You've got the standard "dial in a year" time machines. You've got "time gates" of all kinds - man-made wormhole machines, natural wormholes in space, transient "cracks" like in Time Bandits, etc. For future-only travel, you've got suspended animation and relativistic time dilation. You've also got non-material, mental/information-only time travel, like Quantum Leaping back into other organisms, or transmitting a signal through time that assembles a robot/clone which can receive a mind from elsewhen. You've also got immortality as a way to have an NPC span multiple eras - imagine a villain who encounters the PCs throughout his thousand-year lifetime, piecing together the nonlinear PC timeline as he goes. Reincarnation would work similarly. You could have a machine mind that spans eras, or a hive mind with a long racial memory.
I think a time-hopping campaign that used all of them at different times would be fascinating.