TimeWatch and Paradox (or "what to do when you accidentally shoot your own ancestor.")
One of the big questions for time travel games is "how does it deal with paradox?" For TimeWatch, my answer was "if I can't come back in time to help myself in a fight, how do I even know I'm in a time travel game?" Stupid time tricks were a must, and that meant finding a solution that was easy, memorable, and consistent with GUMSHOE mechanics. I hate referring to rule books during play, so whatever I decided on had to be something you could keep in your head and easily remember.
The answer came from the classic GUMSHOE ability "Stability," used with great success in horror games like Trail of Cthulhu and Esoterrorists. It's traditionally a sanity stand-in, measuring your ability to keep it together when horrible things are happening around you. In a horror movie, when the protagonist starts to scream or twitch or unload their revolver at the shadows, they've probably failed a Stability test.
In TimeWatch this has been hacked into the General ability
Chronal Stability, which measures the degree to which you're anchored in time. When Marty McFly starts fading at the end of Back to the Future, his Chronal Stability has dropped below 0. Drop too far and the universe can erase you from ever existing, or can subsume you and give you a history and personality from whatever time period you're currently in. Time travel risks a little bit of chronal stability, as does paradox. If you do something impossible, like your future self coming back to help you in a fight, you'll be making a Chronal Stability test to weather the paradox. The worse the paradox, the more difficult the test. If you're playing a goofy game, tests are made less severe; if your game is particularly dangerous or punishing to casual paradox, the tests can become more severe.
These points are restored by the Reality Anchor ability, where your allies remind you of who you are and why you're there. An agent with a good Reality Anchor skill can keep the rest of their team nicely grounded, and keep them out of danger as they clock in and out of time.
One Investigative ability addresses paradox as well.
Paradox Prevention is an ability that tells you when things have gone wrong in time. Spending these points can give +3 on a Chronal Stability test, sure, but they let affect reality to a certain extent. This occasionally allows you to bypass paradox and pull wacky stuff out of your hat. If you want to try a cool time trick and the GM thinks it is overpowered, she may allow you if you spend one or more Paradox Prevention points. They're a resource that only renews after each mission completes, so they allow a certain amount of clever time tricks without overbalancing the game.
If your game has parallel timelines, Paradox Prevention points are also spent to travel from one timeline to another parallel world. Some timelines are more difficult to access than others. For instance, TimeWatch foes are usually exiled to a prison parallel that requires one point to enter but 10 to leave. Send a villain there, and it's unlikely you'll ever see them again.
The result? Agents who are cautious and aware of chronal stability throughout the main part of the investigation, but who have the resources to pull out all the stops and use dirty time tricks when the crap hits the fan and their investigation gets messy. Frankly, that's just the way I want it.
TimeWatch is an upcoming Pelgrane Press GUMSHOE RPG about time cops, by Kevin Kulp, due to be Kickstarted in January 2014. Stay in touch on Twitter at @ Timewatchrpg. To be notified when the Kickstarter goes live, click http://bit.ly/1hSd99K