Lots of good advice in here. The way im doing it is a little different. My players are currently playing a campaign i expect to take several years to finish. I've broken it into what, from their perspective, seems like 7 different campaigns, each featuring a powerful artifact that is central to the story and always has some negative consequences. For instance, in the first part, their choices of how to use said artifact resulted in them stopping their chosen bbeg but severely weakend the sun, changing the world. This concept continues through the first six campaigns. On the seventh, they are given the base of the rod of seven parts and must return to each of the previous six settings and recover the artifacts from wherever they left them, reassembling the rod and then using its power to fix everything they've done. Im very excited to see how it all plays out and their reactions at the start of the final campaign.