TIMELINES
Timelines is a game for 3-5 players.  Each player gets to play a different incarnation of Jamie Titor, the inventor of time travel... in multiple timelines.  Unfortunately, most of Jamie's attempts to alter the past have only made things worse.

Inspirations and Points of Reference
“‘All You Zombies--’” the short story by Robert Heinlein.
“By His Bootstraps,” another short story by Robert Heinlein.
The Man Who Folded Himself, the novel by David Gerrold.
The Time Traveler’s Wife, the novel by Audrey Niffenegger.
Primer, the film by Shane Carruth.
Groundhog Day, the film starring Bill Murray.
Edge of Tomorrow, the film starring Tom Cruise.
Back to the Future, the film series starring Micheal J. Fox
All the Damn Time, a Fiasco playset by Will Hindmarch.
Microscope, the roleplaying game by Ben Robbins.
Virtual Play Episode 71, a podcast episode about the Time Watch RPG. 
Fastcast Episode 13 and 14, podcast episodes about time travel and time travel games.

Who is Jamie Titor, and What is The Time Machine?
As a group, collectively figure out who Jamie Titor is.  How old is Jamie?  What's Jamie's gender?  What form does the Time Machine take?  How old is Jamie when the Time Machine is invented?  Just why does Jamie have the skills to invent a time machine anyway?  Answer these questions as a group.

Each player will author a single Regret.  A Regret is a point in time where things went wrong, and Jamie wishes that things could have been different.
Take an index card, write the scene of the Regret, it's date, and put it in the Timeline.
For example, you might have Science Fair, 1999, when Jamie got expelled.

When it's your turn to create a Regret, you can place yours before, after, or in between Regrets already on the Timeline.

What is the Timeline?
The Timeline is a chronological listing of events that have happened in Jamie's history.  They should be laid out on the table in order.  Later in the game, some of these events may change, replaced by other events.  Time travel is like that sometimes.

Let's Start Playing!
Now we're going to play out the scenes of Regrets, in chronological order.

If you created the Regret, you control the scene.

Whoever controls a scene has some key responsibilities.  They get to:
Decide when and where the scene takes place,
Who, other than Jamie is there,
Play Jamie during the scene,
In certain scenes, may be able to declare in advance the outcome of a scene,
Choose when to end the scene.

For the Regret, if you control the scene, you'll declare the outcome of the event- we, as the players and the audience, know in advance what is going to happen wrong for Jamie.  It's our responsibility as a group to make sure that happens.

Once the scene is played out, you might write some notes on the index card.  Include at the bare minimum the following:
Where the scene happened
Which other characters were present in the scene
Any important items that were present in the scene

Let's Add a Time Traveler!
Once all the Regret scenes are played out, pick someone to be the first player.

That player will create the first Jamie- Jamie #1 in fact.  Whenever a player creates an incarnation of Jamie, they write down:
Which incarnation this is (the first one is Jamie #1, then Jamie #2, then Jamie #3)
What year this Jamie is from (write down the year, as it's noted on the index card)
What this Jamie regrets (write down all the Regrets that have occurred up until Jamie figures out time travel.)

Note that once play starts, Jamie of pretty much any age can invent time travel.  You may end up with Jamies of wildly different ages.

If your Jamie feels that a Regret has been fully resolved, cross it off your card.  If something new arises that you feel you regret, write it down on your card.

Let's Time Travel!
Jamie, having just invented time travel, is going to go back in time and fix one of those Regrets so it never happened.  Pick an event on the timeline you want to change, and create a scene that you will control.

For this scene, you'll play your time traveling Jamie.  Someone else will likely play the original, non-time-traveling Jamie, and assign other roles as needed.
For this scene, you get to declare that things will play out a certain way.  Jamie remembers how things happened, and has the opportunity to rewatch the scene, and even to redo it.  Jamie has perfect timing when meddling in a scene- the outcome that you declare will absolutely happen.

For example, maybe Jamie manages to fix the science project that got Jamie expelled from school in the original timeline; for this scene you'll declare in advance that Jamie will win the science fair.

After playing out the scene, take out a new index card, rewrite the description of the scene, with a ' mark after the year.
Science Fair, 1999', where Jamie won first prize.
(This is pronounced “nineteen-ninety-nine prime”)

You can only declare a short term outcome directly related to this scene.  You can't declare changes to any events further down the timeline.

Ripple Effects

After Jamie alters a timeline, for each event further down the timeline, roll a d6, plus all the Paradox dice (see below; at the start of the game, the Paradox dice are 1d4.)

Roll an additional d6 for each element (person, place, thing) shared by the two scenes.
If you think the events are strongly connected, and that altering the one would likely alter the other, roll an additional d6.

If you didn't roll any sixes or any ones, then the event doesn't change- it will play out in this new time line more or less like it did in the previous timeline.
(“Even though I didn't get expelled in 1999, the tech firm still doesn't hire me in 2010.”)

If you rolled any sixes, and didn't roll any ones, then the event on the timeline improves measurably.  Describe how your interference earlier causes this event to resolve better for you.
(“Because I didn't get expelled in 1999, I get that job offer in 2010!”)

If you rolled sixes, but rolled some ones (but not more ones than sixes), then the event on the timeline improves, but not in a way you might expect.  The player to your right will describe how your interference causes this event to resolve differently (not necessarily better or worse, but different.  It might be predominantly better or worse, but it's not the clearly ideal or worse outcome.)
(“Jamie still doesn't get that job offer instead, but because you didn't get expelled in 1999, you get hired by a smaller start-up.”)

If you rolled more ones than sixes, then the event on the timeline is actually worse because of your meddling.  The player to your right will describe how.
(“So the guy you beat in the science fair?  He always suspected you of cheating, but could never prove it.  When you try to get a job in 2010, he manages to use his connections to get you blacklisted.”)

Increment all events further down the timeline.  2010 becomes 2010'.

And then add another d4 to the Paradox dice.

Let's Add More Time Travelers!
You're going to create your own Jamie, just like the last person did.

This Jamie is from the newly created timeline.  Since Jamie #1's original timeline never happened, Jamie #1 technically never existed.  (However Jamie #1 still certainly does exist, somehow.)  Instead it's Jamie #2 (or whoever) that invents time travel in this timeline, with different Regrets and agendas.

Create a Jamie from this new timeline, just like the player before you did.  And then go and pick a scene to meddle with, 'fixing it,' but creating new Ripple effects.

When you alter an event that's already been altered, add another prime to it.  2010' becomes 2010'' (2010 Prime-2, or Prime-Prime if you prefer.)

Back Around to the Beginning
When play comes back around to the first player, you have a choice.  You can continue to play Jamie #1, an exile in time, since your timeline no longer exists.  However, other time traveling Jamies' have been meddling with your timeline, so maybe you still feel the urge to correct it.  Keep playing this version of Jamie, and pick another scene to fix.

Or you might choose to retire this Jamie if you feel this Jamie's goals have been sufficiently completed.  If this happens, create a new Jamie from any point in Jamie's current timeline, just like the last couple of players did.

As the game continues, each player in turn will have this choice: keep playing their Jamie, or make a new one.

A Summary of the Metaphysics of Time Travel

Time travel can be tricky.  Keep these rules in mind when playing the game.

When you time travel, you leave your old timeline and create a new one, that's exactly the same, except now you, an entity from a different timeline is present in it.

There's no going back to the old timeline; this is what you've got.  See Doc's speech in Back to the Future II.  From this point on, you are an exile in time.  For all intents and purposes, your timeline, your history never happened.  You can't change your personal memories or history, but you can hope to do better by this timeline's version of you.

From the point at which you arrived, things are going to necessarily change, sometimes intentionally, sometimes by accident.  This new timeline is flexible; your actions can and will change it.

Time travelers can a technique they call the rewind method and try an action over and over until they get it right.  Maybe you're creating lots of alternate timelines when you do this, maybe you aren't.  It really doesn't matter because only the last one matters.  The reason you get to say how Time Travel scenes play out is that Jamie can make sure the scene plays out the way it's supposed to.

The rewind method doesn't work on the large scale because by the time you realize what the consequences of your actions are, you've already made larger changes to the timeline.  You're not the only time traveler involved, and they'll have a chance to react before you do.

Time travelers can certainly still change a scene that another time traveler has changed.  Only the most recent time traveler can use the rewind method; the earlier time traveler's actions have already happened.

Changing the time line is no guarantee that other people aren't going to want to change it also.  In fact, it probably makes it more likely since you've inserted some paradox into the world.  The other time traveler is probably going to be an alternate version of you: after all, you were smart enough to figure out time travel in the first place, why can't they?

When a timeline changes, any time travelers active in that timeline also follow along to that new timeline.  They show up at the same place and time to wherever they were when time changed.  If it's important for gameplay for some reason, that time traveler decides the when and where.

There's time, and then there's meta-time.  Time is time as we understand it; always going forward.  Of course, time travel lets us move back and forth on that, letting us move four dimensionally.

Meta-time is the inexorable progress of the timeline itself into a different states.  Time travelers move around in meta-time.  Once you change something, you can't go back.  Meta-time is much less understood.  But remember, we're keeping turns, and the game keeps going forward.  If it helps, you might think of each player's turn as a unit of Meta-Time.  If that makes things more confusing, try not to dwell on Meta-time.

If a Jamie retires, it's because they've found a timeline they can live with.  They aren't an active time traveler, so when a new timeline is created and the other time travelers move on, this one stays behind.

Paradox is the measure of how internally consistent the universe is.  Since there are time travelers running around from a different timeline, the universe is inherently inconsistent.  This is why Paradox starts at one, and keeps increasing.

You cannot meaningfully kill a time traveler.  You are either killing a version of them from a timeline that no longer exists, or an earlier version of them will manage to avoid the fate.  If, for example, you try to kill the version of Jamie #1 that meddled at the science fair, you'll do so.  But Jamie #1 is going to be still running around.  If you confront what you believe to be the most current Jamie #1, you might do so, but there's still an infinite number of prior points from which Jamie #1's time traveling existence might continue.

How long do we play?

How do we add more scenes to the timeline?

Should there be other ways to add paradox to the game, or paradox to specific Jamies?

Can you change events from before Jamie's birth?