[thastygliax]

I have two cases of running multiple campaigns in the same setting and using elements of previous campaigns as backgrounds for the new one.

The world that I created for my college AD&D 2E campaign underwent heavy revamping to become the setting of my GURPS Fantasy campaign several years later. The changes were profound enough that I didn't use any PCs from the first game as NPCs in the second one. However, I did later start a D&D 3E campaign based on what I could recall of that 2E game rather than the GURPS version of the world. The PCs of that third game belonged to an Adventurer's Guild founded by PCs from the first one, but the game didn't last long enough to introduce those NPCs "on screen."

I have also run three campaigns set in Green Ronin's Freeport setting (D&D using v.3.0, v.3.5, and v.3.5 but converted to Pathfinder mid-campaign). My wife and one other friend have played in all three games, with new characters each time except for the latter playing the same PC in my second campaign. In the third game, her PC became an NPC sage who the party consulted from time to time. I also used some of the other retired PCs as NPCs in the second and third campaigns, though not to the same extent. Another player from the second game decided to make his old character the hated enemy of his new PC for the most recent game. We ended the campaign before he was powerful enough to challenge his rival, but that showdown is the obvious choice for a one-shot adventure if we ever do a sequel.

In addition to those two examples, I've thought about using some PCs from yet another past v.3.5 campaign as NPCs in my current Pathfinder campaign (which has three of the same players), or possibly as PCs for a high-level one-shot set there. It's a different world, but it would be fairly simple to adjust the characters' histories to fit the new campaign.

I'm also considering running some adventures for my kids (age 13 and 12) set in a different part of my current campaign's world. If I did, there would be a possibility of those PCs meeting (or even joining) the main game's PCs when my children and their characters are older and more experienced. (We might need to convert characters at that point, as I prefer running D&D 5E for my kids.)