A while back (A New World for a New Campaign), I laid out how I thought I/we could develop new campaign worlds. There’s a reason I think about things like that - Fletnern (my current campaign world - available here for FREE!) is over 30 years old. Plans I have had in place for >20 years are still brewing. I just can’t bring myself to dramatically alter the world in a way that I haven’t had planned for at least a decade. Yeah - I know - that’s a personality fault, possibly related to OCD.
But the truth is: I absolutely love “What If” comics. Take characters and settings you know and love and then twist a major plot point. I love those things, but in the back of my mind is that lesson that all creative folks should have learned from the “lost season” of Dallas: No one wants to wake up and find out that Patrick Duffy is alive and in the shower. OK - That wasn’t really the lesson, but you know what I mean. No one wants to invest time and energy into getting involved in a story line just to learn that it was all “a dream”.
I actually have a usable technique for this, though it is a little too comic book based: I use alternate realities. Here’s the reason - In an alternate reality/timeline, you can change things, tweak the past to develop a whole new future, and then return home without having that timeline affect your main campaign. BUT! You can also return to that timeline at some point, so it doesn’t really go away. It is like having a series of pocket campaign worlds where you don’t have to redraw the maps.
You can of course do it any way you want, but I would greatly limit who can jump the time streams. In my world there are really only two ways this has happened: 1) there is a titan who can move from one timeline to another and he takes an interest in the people who are critical to various time streams (read “the adventuring party”) and 2) I have allowed an ancient ogre/goblin/orc spell to do it. Before you misunderstand this “spell” - it is based on the style of magic called Ceremonial Dance. If you know the history of the Ghost Dance, this might sound somewhat familiar. To get up enough magical energy to make the spell work, you need to get entire villages and towns of people performing the same magical dance together. Though I never defined it all that well, it takes well over 1,000 spell casters working together in an enclave style.
The titan is a lot more fun, because he takes people with him. He reports back on what your alternative selfs are doing. Since half the timelines move forward (as we live) and half move backwards (not that they accept they are backwards, they insist you are living backwards), he doesn’t actually time travel. He slips into the various time streams and until he winds up where he wants to in yours, then transfers out. So in order to travel back 120 years, he has to wait 120 years in an alternate time line. Since there are currently at least two of these guys running around, at least one of them learned how to magically hibernate, so he doesn’t actually live through everything, though he has less detailed information than his alternate twin.
This is already way too long, but why do it? Ever make up an enemy or enemy group that you loved, but your players made mincemeat out of? Well, in an alternate time line, the party failed and these guys got their way. Now the “only group to have defeated them” needs to time slip in and kick their asses again, only this time, they are vastly more powerful having been successful in building up their base. Were they on the winning side of a war? Alternate timeline - they lost, and now the last surviving member of the party has come to beg for aid. (I’d use a loyal follower here and not actually one of the player characters.) Yes - This is pretty Days of Future Past, but it is also Star Trek, Dr. Who, and generically every other sci-fi show at some point. Might even think Terminator.
I think it is very important to let them know that this isn’t their reality. Maybe not at step 1, but definitely early on. Otherwise they feel like their characters were written into a bad Dallas season.