Well, on the off chance that it'll help someone here with ideas, let me spill out a concept I had for timeline/reality modification I had once for a superhero RPG I was running (well, a rather dark superhero game where heroes discovered the source of their otherworldly powers were a form of energy that their 'mutant' genes were an antenna for and that energy was slowly damaging reality - breaking it down and making it dissolve). But for a while, I was flirting with the idea that Grandfather's pinched off pocket universe was all about studying the nature of reality as a hook to reuse the concepts for a new group of players.
A campaign could be made around the players stumbling on Grandfather's pocket universe and given an oppurtunity to speak with this godlike being. And perhaps, out of whim or even pity, he allows them to play with his reality lens some. Or more likely, he feigns pity because he's curious to see what these devolved beings do with his toy - he might have moved on to even more interesting things, things that make reality travel look mundane and we couldn't even comphrehend a fraction of.
The Reality Lens in Traveller would probably be some device that doesn't fit on the TL scale. When you discover it, TL essentially becomes irrelevant because, in the words of one of my players from that past campaign, you've "just found the screwdriver that God used when He put the universe together." It essentially uses psionics to view images of other realities (psionics in this case being a manipulation of extra-dimensional particles, like how some modern physicists view gravity). Travel to these other universes is much more difficult, but in this case, Grandfather has figured out how (it would involve a predictably bright flash of light as energy is 'sucked' from the target reality to compensate for what the characters are bringing).
Possible campaigns might be of players trying to 'collide' universes together to create an ideal reality and eliminate unpleasant ones by influencing events. Not only that, they could run into different versions of themselves doing the exact same thing, except with different results in mind, leading to indirect conflict where they know of their doubles, but can never meet them, since they don't know which way Synchronisity will shuffle if they met (or perhaps they know the other is stronger so spend their time trying to achieve their goals and avoid themselves).
There's a number of rules and tenets to how this "time travel" works:
1. Reality is organized something like a "tree" with different decisions branching off of the main branch as they're made. It's entirely possible that there is a "true" trunk to the tree of reality somewhere from which all other realities branch off from. What is in this "true" world is up to debate. It is like some place like Zelazny's "Amber"? Is it where the Creator dwells? Nobody really knows, or if there really even is a central "trunk." It's something of a religious/philosphical debate amongst those in the know.
2. Of course, this means that every picosecond (or less) an infinite variety of realities are being created by the "decisions" of every bit of energy. However, the main branches of reality have a certain gravity to them: If the differences aren't large enough to generate a "break-away velocity" the branch is immediately sucked back into the main branch and resynchronized. Those living in that reality never even notice the difference - for "us" every moment of our existence we're being shuffled in with slightly different versions of ourselves. As certain events are more likely to happen than others, a sort of "mob rule" decides the nature of this 'master reality.' This effect is known as Synchronisity. Synchronisity also has another effect, detailed below.
3. The less alike a reality is from yours, the more difficult it is to travel there. So a world where magic works would be almost impossible to travel to from most TUs. However, you could 'inch forward' by going to intermediate realities between between your reality and the other.
4. If a reality is too much like yours, you face Synchronisity or an inability to target it. Realities that are too close to your own can't be targetted because they're still too alike your own reality. The targetter of your "time travel device" keeps getting reset to your own by Synchronisity effects.
5. Branching realities experience time at different rates. Some realities split off and move at a slower pace, others faster. This is the most important and most interesting point for "time travellers." It's possible to "travel through time" by going to a reality that is moving slower than your own to go into the "past" and faster than your own to travel to the "future." However, you're not travelling to the past or future in either case. You're travelling to a past for future.
A few observations:
You're not really time-travelling, though it looks like it. Because of the nature of reality travel, it's not true time travel, but for those experiencing it, it might as well be. So if the players want to go "back" in time and prevent Strephon's death, for instance, they're welcome to. There's an infinite number of realities where this has already happened.
No Grandfather Paradoxes. You don't get this problem here. Without the egotism of the "one true reality" existing, you can have as many permutations of reality as you'd like. If a kid finds a "slow" reality and kills his grandfather, that reality will simply go on as a place without grandfather (or descendants). Of course, there'll also be a few branchings where the gun doesn't work. Where the kid uses a knife and is caught. Or the blade breaks. But most of these realities are folded back instantly as unlikely.
Be Aware of Synchronisity. Due to the nature of reality to "fold in" things into itself, there's a danger to travellers that they'll get "folded in" to the reality they're visiting. It's the most extreme in cases where another version of you is already alive. You get synchronized into the version that's already there. In most cases, all differences are "lost in the shuffle." GMs may choose to have certain events stick around as incongrous thoughts or deja vu and such. If a version of you was once alive, you'll get synchronized with the memories of yourself as you once were - your personality will shift towards the former you. So if in this alternate reality you were "evil", you would gradually gain the memories and personality of the alternate you. Synchronisity isn't always bad. Most arrivals to any new reality are "brought up to speed" (or slowed down) to the local time scale, as well as any other smaller changes to allow them to operate. Those who don't go insane.
There Are A Nearly Infinite, But Finite Number of Realities. Due to Synchronisity effects following permutations of large "probable" events only, and folding all others into itself, there are a finite number of realities. Mind you, this number is so large, we humans have yet to find a need to write out a number that large, but it's not infinite. It's also entirely possible for a formerly independent reality to get folded in if it becomes too similar to another, with the reality with more probable events being dominant. Again, those who dwell in both realities never notice the difference when folded together (with the GM's option that a few individuals might realize something is different - this isn't a good thing and usually leads to madness. It's like when Krishna shows Arjuna the nature of reality in the Bhagavad Gita, and let's face it, most people aren't as enlightened as Arjuna).
Madness Is Relative. This is one of the disturbing realizations that players will make as they travel - that many of those with mental problems could just as easily be considered as "de-synchronized" from the reality they're in, but perfectly fine in another. The biochemical and genetic imbalances and all that are there, but those aren't always problems in other realities.
You Can't Go Home - Home Is Where the Heart Is. By this view of reality, players who leave their reality can never return and be sure that the reality they return to is really the one they left. However, pragmatic characters will realize all they need to find is a close-match and Synchronisity effects will shuffle them into their new reality and they'll never know any different after that. This will probably disturb a lot of players, but hey, when you're playing around in the Creator's Toolbox, it's inevitable you discover some unpleasant Truths (with the capital "T").
You Can Find The Ultimate Universe. This is probably the greatest temptation of all reality travellers. It's possible, if they look long enough. They can find that universe where not only did Strephon live, he goes on to do all the things the players would like him to do, in addition to that, the character marries his (or her) one True Love - the one he or she loved and lost forever. And better than that, the character and just happened to be vaporized in anti-matter explosion a moment before...allowing the player character to step in and make things "right" again. Or perhaps unmaking a decision he or she really regretted. This possibility can be mined endlessly for pathos (for a suprisingly good example of this in an otherwise cheesy movie, see Kirk in the "Nexus in Star Trek: Generations" - he knows it's not "real" but he doesn't care).
- Sample Idea: One of the characters in my current MT campaign is like that, playing the concept of the "regretful war criminal / concentration camp guard." He was one of Lucan's Admirals in the Final War. The "reality" where my campaign is in is the one where he's basically "retired" to. So he knows things he shouldn't, and he occasionally slips with knowledge about the future and such. He was the architect of the worst of the Black War strikes, responsible for orders that killed millions directly, trillions indirectly. He's also heavily irradiated from the Final War in his reality, so he's suffering chronic and progressive tissue damage that an obscenely expensive cocktail of drugs and treatments can only slow, not stop so he'll die soon. He knew about the Virus and when Dulinor attacked the Research Station, he knew it was the end, and started searching for a reality where he was killed but the Civil War was going on. He was determined to blunt the effects. But he doesn't mind - he sees it as fate/karma/God catching up to him. He spends his time helping charities and trying to stop the bloodshed of the Civil War in the time he has left. All the other characters think he's this walking saint (who coughs up blood) ... if they only knew. Now that their thermonuclear bombardment of Cymbelene failed to destroy the chips, they're off to strike at Omicron station and our admiral is hitting Synchronisity head-on and he's going to lose. He's realizing that he's changing the lyrics to the song, but the melody remains the same.
Reality is Relative. Reality is based on belief. Ordinary sophonts can't really tap much psionic power, but when you add trillions of them together, they decide what is "likely" and what isn't. This is a clause that prevents players from taking fusion guns to conquer a stone age Terra to change reality, for instance. If the local reality isn't ready to handle your fusion gun ... it simply doesn't work. No amount of analysis will ever figure out the problem. Chemical reactions that are laws where you come from simply don't work in your weapon.
