Time. The ever-onward, eternally turning wheel. Often in games the branch of Time Magic is given a level of power unequaled by any of the other branches of magic. Time magic is one of the only schools of magic that most GMs outright ban within their games. And rightfully so, as messing with time can produce horrifying results, both in terms of the game world and gamemastering.
Once the players can manipulate time, what's to prevent them from eliminating your prized super villain, reducing your carefully crafted campaign to a single cold-hearted adventure of time-hopping genocide? Kill the baby before he threatens the world. Better yet, kill the mother too, just in case she produces another child that turns into the Antichrist. Pretty soon your characters are starring in their own remake of Terminator, except they don't have an Austrian accent going for them.
Plenty of games have portrayed the results of time travel, as well as a number of movies and a few t.v. shows. The common theme is the same in all of them, however: time must not be changed, or the world and future will suffer in unimaginable ways.
So, keeping in mind such wonderful movies like Terminator and The Time Machine, and video games such as Chrono Trigger and World of Warcraft, what would the general result of time travel be within your game world? Who would be able to travel the streams of time? What would they do? Would a time-watcher organization pop up to manage things?
What would your world be if time travel were possible?