I have been thinking a lot about time travel lately as it relates to portals, teleporting and dimensional shifting.
One of the things I have been kicking around is the idea of linked portals both advancing through time. The simplest example is two portals linked together and advancing through time at the same rate. So you step through a portal and pop out five years in the past (these portals may or may not also travel through physical space or dimensional space, regardless of being linked temporally). You spend a year farting around in the past, then step back through the portal and find that you have missed a year in your original timeline.
A slight wrinkle is to have the two portals advance through time at different rates. This is what happened recently in my Mutant Future campaign, when despite experiencing just a few weeks as the party experienced time, the characters found that almost two years had passed when they passed back through a dimensional portal into their original home (and home dimension). This allowed me to make some changes to the swampy home they have come to love, such as the disturbing tendency of recently killed things to rise up from the dead and stumble around looking for brains, and some major reshuffling of the local power players (most noticeably the Knights of Genetic Purity withdrawing from the valley and a major increase in Julius Corple's activities and influence).
Anyone else ever played around with this? I first got the idea when a new character in the campaign got the Plane Shift mutation and I was coming up with a list of some random destinations in case it was ever used. One of the destinations I wrote down was in the past of the campaign world, before the apocalypse.
That destination came into play because I just couldn't resist it... I had decided that the first time the mutation was used, it was opening to the present. I had gleeful visions of a mutant party crashing through Eugene, looting the army installation by the fair grounds and then jetting with the military in hot pursuit. The first time the mutation was used, the party was in a desperate struggle against a Xiticix Killer (a great monster that I stole from a wonderful Rifts book called Lone Star which I actually wrote a review of on this blog - I highly recommend Rifts as inspiration for Mutant Future DM's, even if you just look at the pictures...)
In a sudden flash of inspiration, the player with Plane Shift opened a portal to random destination behind the creature and the rest of the party bull rushed it and forced it into the portal... to a busy intersection in west Eugene on the evening that I was playing the game with my friends in real life. Only of course, we didn't hear about the national guard being called in to deal with an alien monster that night...
I recently picked up Transdimensional Teenage Mutant Ninja Turtles, which is more or less an entire book of possible destinations for time portals as well as advice for running exactly the kind of time and dimension-spanning game that I have come to love!
So of course, I recently had the players discover a "portal tree" with 18 twisted columns of energy intertwining around each other, each leading to a different destination if stepped into.
They have learned what is on the other side of about half of the portals. They very nearly dropped everything else they were doing and went exploring a giant temple of the snake men on an apparently abandoned planet, and they stuck their heads out in my 4e campaign world (two players who cross-over between my campaigns were duly impressed, and got to feel like they were in on a joke). But what is on the other side of the other nine portals? Or when?