4E has taken the time to explain the Scope and Scale of a campaign and detailed this as Tiers: Heroic, Paragon and Epic. Personally I like this method as it helps define a campaign and make sure it progresses. However, I think their examples of Scope can and probably should be defined differently.
Let’s first talk about what I mean by Scale. Scale is mostly mechanic. It is the levels of the monsters and the abilities they have access to. As a campaign progresses this Scale increases so you are fighting monsters capable of continuing to challenge the player characters. The campaign scales up in response to the increase in player character levels.
In 4E this means the adversaries now have access to more abilities, in addition to increased combat numbers. At Paragon Tier they usually have an extra encounter power and at Epic another one and probably another daily power. Also at Epic Tier a lot of the monsters start having abilities that negate some of the player character’s abilities to “lock” them down.
For Scope, I mean the things they encounter in relation to the campaign setting and this is where I recommend a divergence from the ideas put forth in the Core books.
The DM’s Guide defines Heroic Tier as village-sized; this is where the PCs encounter “small” challenges that do not affect much beyond a village or city level within the setting. Paragon moves the PCs into adventures that affect nations. At the start of the Epic Tier they recommend moving this Scope up to world affecting adventures. Then as the campaign reaches the end of the Epic Tier, adventures focus on fighting gods and saving the universe.
I say it is possible to reduce the Scope of a campaign. There is no reason why the Heroic Tier can not remain in one place as normal; perhaps investigating a local forest. Done properly this could easily occupy the PCs all the way through Heroic Tier. At Paragon Tier the PCs now have to save the local duchy or larger city. With a good layering of intrigue and the standard dungeon crawls it would be easy to keep the PCs in that one location until Epic Tier. And once they get to Epic Tier they would be saving the kingdom from invasion or corruption. They would never have to save the world or leave the planet.
In fact, 4E is actually designed to keep the PCs at this lower Scope. Even at the highest Scale (level 30) they still have a reduced Scope. They can not move mountains, their breath does not knock over a forest, they can not leap over oceans. They are not gods. The PCs actually function at a much lower Scope.
However, how do you reconcile Scope with Scale? Most published Epic adventures feature gods, demon lords and elder dragons. It makes sense that these types of adversaries are of Epic Scale and Scope. However, because of the diminished Scope the PCs will not be fighting these types of monsters. They will instead be fighting things that also fall into a smaller Scope.
And herein lays the quandary. It seems contrary to have level 30 PCs fighting goblins (it does not feel right in Scope). Likewise, it would not make sense if there was a level 30 goblin running around the world; it runs against our biases and “logic” as gamers (it does not feel right in Scale). In this case, Scope (Epic) and Scale (Level 30) would not mesh up. A DM needs to find adversaries that are Epic in Scale while still falling within the Scope of the campaign.
One way to do this is to keep to adversaries that are similar to the PCs. If it makes sense that the PCs are level 30, with all of the abilities therein, then it also makes sense that another member of that race also would have those or similar abilities. It is easier for a player to reconcile fighting a troop of level 30 human knights than a band of level 30 goblins.
Another is to stick to variable level monsters. Dragons run across a broad range of levels, so fighting one at level 10 and another at level 30 makes sense. It does not have to be a world-eating elder dragon, but rather a venerable dragon that has slept for the past 200 years.
A side option here is to “save” these variable level monsters for higher Scale and Scope. Don’t throw a dragon against them at level 10 even though you have monsters stats for them at that level. “Save” the dragons as encounters until level 25+. At level 10 throw elite goblins at the PCs.
With this diminished style of Scope it is possible to run “grittier” campaigns instead of the high fantasy 4E seems to have as a default. Scope does not have to end with universe shattering adventures. It can still remain “small” within the campaign setting.