Oddysey asked a question about returning to an old but beloved campaign a few days ago. I haven’t had the time to write the sort of pithy and succinct response appropriate to comments, so you get a long and rambling dissertation. ;)
”Which leaves me wondering why it is that they want to play this sequel game so badly.” - Oddysey
Very good question. The answer should guide your next steps.
I'm going to go waaay out on a limb here and suggest character empathy is a strong part of it. Your players feel a really strong connection to their characters, and they want to see how the story goes on, what happens next. (If this is the case, congrats! You did an excellent job of GMing,and they did an excellent job of playing.) Beloved characters are a joy to play just in their own right. When they already have a collection of cool gear and a web of associations with interesting NPCs, there’s even more fun in the offing.
If this is true, you really need to let the players either play their old, beloved PCs or new PCs who have a strong connection to those PCs, like their children or close retainers and lieutenants. In either case, you’re not starting the campaign at the traditional 1st-level-nobodies. Even if the PCs are first level, they’ll likely have access to powerful contacts and resources. You can, of course, invoke some major disaster to sever the PCs from most of these, a la the opening paragraphs of A Game of Thrones. However, this isn’t necessary and might feel a bit like a cop-out. The “Road Warrior” is as great as it is primarily because it doesn’t bear much resemblance to “Mad Max”. Likewise, “The Empire Strikes Back” doesn’t start with Luke back on the farm, but picks up where “Star Wars” left off.
You might, however, have serious issues if your players feel too strong a connection to the characters. If they’re feeling overly proprietary, and absolutely refuse to allow anything bad to happen to their beloved PCs or associated NPCs, it may not be worth revisiting that campaign. Give your imprimatur to whatever fanfic they want to write, or just tell them that everyone lived happily ever after, and leave it at that.
Another strong draw may have been the themes of the campaign. If your campaign was all about fighting for social justice, or dealing with a doomed world in an unfeeling and alien universe, or had a strong comedic element that really resonated with your players, this may be what they are looking to return to and play with. If this is the case, you might be better off with a new campaign, or maybe even an entirely different game system! However, you’ll have a hard time convincing the players of this if they haven’t made this realization themselves.
Finally, the players may feel that the campaign ended without resolving issues that they feel personally invested in. This is a not uncommon problem in more sandboxy games where some goals end up being deferred as other take precedence. Yes, Khanthrax the rakshasa prince was defeated and his plot to turn all the good humanoids of the world into rodents was thwarted, but a pretender may still sit on the throne of Lorany. Or Sir Martin never reforged his father’s broken sword. Or Kalit never finished building that temple as she promised Isis she would. If the issue is small enough and personal enough, you might not need a full-scale campaign. A few adventures or scenes might be enough to create appropriate closure for your players. Feel free, though, to let players whose PCs were not directly involved to take part. Friends helping each other through these sorts of things is a rich vein in literature and can be fun to play with, if those other players don’t mind their PCs playing second-fiddle for a bit.
For myself, I’ve both played in such a campaign (specifically, the children and students of the previous generation) and run such campaigns. Some of my old college group want very much to dust off those characters and continue their adventures. The trick, for me at least, is to find new themes to explore and challenges that I find compelling without doing too much violence to the things that the players loved in the original campaign.