Idea 1: Contrive things somehow such that they end up with the PCs left (or stuck) in command of a large army in wartime e.g. a couple of combined Roman Legions fighting the Gauls. The challenges for the PCs then change dramatically: it's no longer about how they themselves can go out and win battles, it's instead about how to keep this army alive and fed and functional and cohesive such that it can go out and win battles. The PCs' concerns become supplies, logistics, chains of command, discipline, morale, and so forth.
When it comes to the actual battles, have the opposition also have high-level help with the idea being that their high-level help and the PCs largely cancel each other out, such that as far as possible the outcome rests on how well the PCs have treated/trained/prepared their army rather than on the abiities of the PCs themselves.
This won't stay fresh for long, but as a change of pace it should be able to give you the equivalent of an adventure or two.
Idea 2: If you're at 20th level anyway you're campaign is probably in or approaching its endgame, and this opens up some new possibilities. You no longer have to worry too much about whether the PCs continue as a cohesive group, for example, if the game's winding down anyway.
This opens up the option of putting the PCs or their goals somewhat more directly in conflict with each other, and of introducing elements toward this end. This would of course depend on the specifics of each PC and each campaign, but say if one PC has a goal or mission statement something like "Defend the monarchy to the death" and another has a goal of "Someday I will sit on a throne as a monarch" it shouldn't be too hard to get those goals to work directly against each other.
(the one who wants to be Queen is secretly approached by a band of usurpers who see her as their leader and replacement monarch; the one who's sworn to defend the monarchy hears quiet whispers of a secret plot against the King, etc.)
You'd need to find a way to somehow make this relevant to each PC, but after that you don't need to do much other than stir things up now and then, as they'll sort it out among themselves. And if they end up splitting apart as a party over this, so what - the campaign was ending anyway.
