You’ve been running a game with the same group of people for years, now. You’ve done your best to play weekly, but it has still taken the better part of a decade to get from the beginning of the game to the end. Maybe the end was a total party wipe because the fighter did something stupid. Maybe your players resolved all the open plot hooks they were interested in and, after amassing an incredible amount of wealth, have decided to retire. Maybe they finally killed that demon/elder dragon/Tarrasque and they’re officially so strong not even the gods would lightly make a move on them. Whatever the reason, the game as you know it has come to an end. Except no one wants it to end because they all get along, it’s tons of fun to play tabletop games with people, and they had this idea for a character they made a while ago that they’re dying to try…
So you extend the game. If the party-wiped, you’ve got a couple of really fun options and one simpler option. If you want to just keep it all going, then you can have some kind of fail-safe alert a new group of adventurers that the people previously trying to solve some big problem died. They get sent out to collect corpses (maybe revive the players who want to stick with their old characters), and then they carry on in the stead of the previous group. It’s easy, it makes sense in a lot of scenarios, and it makes it easy to get down to playing again. If you want something a little more challenge but that would add some depth to your world, start the party over. Everyone rolls up a new character, maybe not level 1, but probably at a lower level, and a new adventure starts. Whatever the old group was trying to prevent has come to pass during the intervening years (any number is fine, but I like to make sure it’s at least a couple of generations so everyone has a chance to discover all of the ramifications of their failure) and the new group is trying to either fix the problem or facing some new threat in the world created by the death of the older group. This, of course, necessitates that the issue the old party was trying to prevent wasn’t something truly world-ending. If that’s the case, you could always throw out some kind of “incarnation cycle” spin and have the players basically play themselves reincarnated on the new world the gods made in response to the destruction of the old world. There’s a lot of fun opportunities their, including relics from the old world and maybe some kind of special, inherited powers from your previous incarnations. The sky is the limit here.
If all of the characters have decided to retire from their lives of adventuring with their dubiously gotten gains, that opens the door for a generation-spanning game! Maybe the players can roll up the children, adopted or biological, of their old characters. Or, perhaps, the child of someone else’s character. Students or protégés are also fair game. However it happened, they’re playing someone who grew up under the tutelage of a character from the old game and, as a result of something happening (anything from the tragic death of their mentor to a decision to emulate their mentor’s life of adventure), has hit the road to find our what’s on the other side of the horizon/save the world/strike it rich by looting the long-dead corpses of other adventurers and the private homes of the various races who don’t live in the same kind of societies that your adventurers grew up in. Maybe an unresolved plot hook the previous generation chose to ignore has come calling again, perhaps grown more urgent as a result of the passage of time. Maybe one of the magic items or artifacts collected by the previous generation is the key to some plot a new villain has hatched and they used the old hero’s advanced age as an opportunity to put their dastardly plan into motion.
If your players have all gotten to the point where there is nothing left to truly challenge them besides the gods, maybe just start a new campaign in their shadows. The new characters grew up in a world forever changed by the actions of the old characters and are inspired to set out on their own adventures by the legends still living and walking on the mortal plane. This creates a lot of narrative fun for the DM because whatever problems the new characters are given to solve, whatever legends they chase, can’t be big enough to draw the attention of the more powerful adventurers who still live and exert their will upon the world. This can also create a lot of fun situations because you can have players reprise their old characters in role-playing moments, maybe because the new characters are hirelings who the old character is paying to take care of some problem that’s probably beneath their notice or that they don’t really have the time to solve on their own. Or that they just don’t want to deal with, similarly to how most people pay someone to change the oil in their car rather than learn how to do it themselves. Whatever route you choose, it’ll be memorable so long as the incredibly powerful previous characters are still around to pop up now and then. Plus, most players love to see their old characters crop up in a campaign.
Whatever you decide to do, just make sure you talk to the players about it beforehand. Most of them would love to figure out what their characters decided to do after retiring or getting too powerful to be stopped, so that’s a good opportunity for them to become more invested in whatever game comes next. The idea of playing in a world where your old character is walking around and living their life is incredibly inviting. The opportunity to maybe run into them and to see them play out a scenario again is one I, personally, would never pass up.