One problem some campaigns run into is Power Creep. Whatever the power-level the PCs started out at, they have gained in capabilities until not only are the opponents they originally faced beneath them, but opponents that would challenge them strain the verisimilitude of the setting. This dilemma is practically designed into D&D, but it can occur in nearly any system and setting…even ones that don’t allow PCs to personally advance in abilities or do so at a tiny rate will almost always allow PCs to advance in political and social power through their connections and influence on NPCs that they’ve befriended and aided over the course of the campaign.
Often the game is supposed to transform at that point. In D&D, for instance, once the characters reached a certain level, it was expected that they would build a stronghold of an appropriate type for their class, and the game would then focus on them dealing with ruling and expanding their lands. Even if there isn’t a set of explicit rules for it, the PCs may find that through roleplay they’ve climbed to the top of their hierarchy, and they’re now in charge.
Their personal level of power matters a whole lot less, and the problems that they face are a whole lot bigger and more diffuse. But for a lot of players, that’s not what they signed on for. Becoming the head of the thieves guild, or ruler of the kingdom, or whatever, was a good long-distance goal, but they what they want to play is a game where they’re James Bond, not a game where they’re M… the fact that they could actually make progress towards the goal and eventually reach it was much appreciated as it was occurring–a distant goal that never gets closer is usually either forgotten or becomes frustrating–but actually playing it out isn’t of interest to them as an ongoing concern.
One approach is, of course, to retire that character and start with a new one. That can actually be pretty neat, and can provide a lot of depth to the setting as you revisit it with a new character’s eyes. But players might not be satisfied with starting over, particularly if the climb has been long and arduous. They can feel like now that they’ve arrived, they should be able to enjoy the fruits of their labors.
Another approach is to increase the scope of the campaign. If they’re not interested in politics and being in charge, and want to keep things on the level of personal adventure, many settings are ripe for out-of-setting travel once you reach Epic levels. Whether it’s extra-planar travel in D&D, or finally getting a working starship and getting off the backwater world you’ve been adventuring on, you have a potentially limitless source of ever-increasing potential danger levels there, without ever having verisimilitude problems with those levels of power in the original setting. Even in a more down-to-Earth setting, you can go from city-bound shamus to globe-trotting detective…if the players are willing to adapt to a slight shift in tone.
Otherwise, you can start to provide players with strategic instead of tactical challenges. Even if it’s a given that they can win any individual battle, there’s nothing at all bizarre or contrived with the notion that there may be multiple places where they want to intervene but they can’t be everywhere at once. Unless you’ve given them access to time-manipulation, time is a resource that constrains everyone. In D&D or such a fantasy setting, the smaller estimates of the size of medieval battles would be 20,000 men on a side…on the high side it might be 70 to 120 thousand or more. Even if they can kill 100 hobgoblins without breaking a sweat, even if they could eventually take on the whole army, that’s more than enough opposition to justify them needing to be in two, three, dozens of places at once if they want to protect everything that’s important to them. In modern or futuristic settings, it’s even easier to justify them being opposed by organizations that have enough manpower and resources that simply winning every melee or firefight can never be sufficient.
The point isn’t to beat them up and take away everything they hold dear, but to force them to make choices and be clever about deploying their power to accomplish their strategic goals and not just think in terms of winning each skirmish they get involved in. Strategic goals don’t have to mean military ones, just that’s an easy example. What do the characters want to accomplish? What do they want to protect? Make them think about that instead of just defeating the monsters and mooks in front of them, and I think it would immediately become clear that they don’t have nearly enough power to make that trivial. Not even Superman can be everywhere at once, so even in, or especially in, a super-hero game the GM isn’t stuck with ramping up the power of the bad-guys and piling on the world-shattering threats until the players wonder how there are any civilians left.
You can also introduce things that can’t be solved by combat or a couple of quick spells. Suppose you’re playing D&D and a plague or famine hits the kingdom…not one caused by a bad-guy who can be defeated, just a natural disaster. There’s not a lot that even a 5th or 6th level spell can do about that directly. Even a 14th Level Cleric can only create food for 96 people per casting of Create Food, 3 times a day…. The players would really have to think about whether there’s a way to use their great power to accomplish something, perhaps by bringing in food from elsewhere, or helping people emigrate. I think it’s helpful for this sort of thing not to have a solution in mind, so it doesn’t become a game of guess the GM’s clever way out. Pose a problem out of the myriad that have plagued mankind since the dawn of time, and if they can come up with a reasonable solution, or at least a way of mitigating it, great! If not, ah, well, hard luck, but it’ll eventually resolve itself one way or another…even if that means mass death or migration, and that can be a springboard for future problems.
Now, not all players are going to like that, and some will down-right hate it. If that’s the case for your players, then I think opening the scope by providing a bigger pond for them to play in such as by planar travel is really the way to go.
One last piece of advice is that you probably shouldn’t try to deal with Power Creep by stripping the PCs of power without discussing it with the players first, no matter how well-justified the take-away is by the setting and system. Hitting your D&D PCs with a bunch of level draining Vampires, or having their home base destroyed by some arch-rival while they were off on an expedition, even if it returns the game to the “sweet spot” of power levels where all the players were having fun, is the kind of thing that can end campaigns. If you think that the PCs are too powerful for the setting and you want to depower them a bit or a lot, perhaps because you didn’t realize what a campaign changer it would be when you let them get the McGuffin of Magnificence or they hit a high enough level to cast that spell, ask them what they think. Do they want to change the nature of the campaign, just the scope, retire the characters with a “job well done”, or will they go for a great dramatic reversal of fortune? Only your players know what will please them best.