I have a fully involved world. Stuff happens all the time, and story lines come and go. Sometimes I get a really cool idea for a story line, but I’m not willing to do it because it would force me to completely rewrite the world. But I have an idea. So here’s my idea: Some of you may see this as a mix of two story lines - one recent cult classic and one legend.
Eons ago, two demigods fought it out on the mortal plane. The battle ended when one cut the other in two, imbedding his sword into the side of a mountain. The seers foretold that only one of divine birth would be able to remove the sword, but when they did, it would cause the end of the world. The real problem is that due to the mysticism involved, removing the sword from the mountain will cause a divergent reality to form. If this tangent time stream isn’t destroyed, it could mean the end of the world.
It took centuries, but a mercenary general convinced an angel that if she were to pull the sword from the mountain for him, he would use it to bring good to the world. Angels can sometimes be suckers. Powered by the sword and the divergent time stream, the general starts a world war - starts and is winning!
The characters get involved because they start fighting the war. Eventually the angel explains to them that if they don’t get the sword back into the hole in the mountain, the world will come to an end, but if they do return the sword key, then this time stream will cease to exist. (Maybe returning the sword will destroy the angel as well, but she is willing to sacrifice herself for that. Is the party willing to sacrifice her?)
So this “mini-campaign” starts with the party getting involved in some minor skirmishes with the mercenary army, which grow as the war becomes a full on world war. Every mission you can think of revolving around wars and religions can work here, including some massive battles taking place, right up until they have to battle the general and get the sword away from him, likely right at the mountain. So as the time tear warps reality and the angel and the general battle over the sword, with tornadoes and other phenomenon, the party ultimately gets the sword and puts it back into mountain. The divergent time streams get sucked back into each other (violently), and the party finds themselves standing over the sword with the general and the angel both dead at their feet. It is now months ago - the date that the general originally obtained the sword.
So cool, right? You can have a massive world war, and then get a mulligan on it. I would let the party remember everything that happened (thus keeping the experience). They might also learn things during the tangent time stream - secret crimes that people have committed or secret treasure troves that they can take advantage of. Others who were closely involved might have dreams of what happened, but they wouldn’t believe the whole alternate time theory. Be careful though. You might all be too young to remember, but people got really mad when an entire season of Dallas turned out to be a dream. Your players might love this whole concept or they might feel cheated for having saved the world, but not getting any credit for it.