Go for a planar field trip.
The PCs are suped up, granted, but then many planar beings are pre-suped up before adding on any class levels. That is the big mistake of dealing with planar beings in general. All of them should have a few class levels. A Balor without levels is little more than a Balor Commoner 1 (or perhaps Warrior 1). Give it, say, 10 or 15 levels in Fighter and it becomes a bit more interesting. Or maybe 3 levels in fighter, 3 levels in sorcerer, and about 5-10 levels in the Eldritch Knight PrC. What about a Pit Fiend Rogue 5 / Assassin 5 / Shadowdancer 3?
Once they've reached epic levels, its time to leave the playground (ie: the Prime) and move on to a setting where their powers are not quite so impressive. I'm sure there must be feats - perhaps epic feats - that allow spells &/or special abilities to bypass little things like Energy Immunity, Critical Immunity, Magic Immunity (or high SR), Mind Blank, True Sight, etc. If not, just rule them in - as Monster feats, so your players cannot (immediately) make use of them. Or perhaps an epic plague has been unleashed on faerun that has SR vs remove disease or other attempts at curing it. Worse, it reduces the SR of all those infected, and it is epically difficult to not succumb to it once infected. The PCs are called upon to help out . . . .
Or perhaps that vast glacier up north melts suddenly, causing a massive flood that wipes clean half of the landscape north of the lake of steam - and the PCs have to help with the aftermath - searching for survivors, rare artifacts or relics now lost, etc. Due to AO's decrees the deities cannot hop down to the Prime to help out, so it is up to the PCs and NPCs to help rebuild the northern half of the map. But the Phaerimm and Shades are taking advantage of the situation to try to spread and rebuild their former empires, and the Drow are starting to arise again (those that weren't flooded out). Perhaps the flood, in fact, is forcing many beings formerly underground to come up - as perhaps as much as half of the underdrak is temporarily flooded for the next few months or years - if not permanently. So mindflayers, Ilmaskeri (spelling?), etc are all arising to the surface, fighting each other on the way and while now above, and at the same time fighting those already on the surface - or seeking to spread into "their" areas. Maybe the Mulharand will be willing to help - or perhaps they are too busy maintaining their borders against incursions. Perhaps the flood washed away sediments covering ancient cities long forgotten.
See how a single event - if major enough - can all but write a potential campaign hook or three? Cormyr, Ice Dale, Waterdeep, etc - all are gone or so terribly damaged as to be in severe disrepair for years or decades to come. Halrua and many southern lands are now amongst the most advanced (along with the Mulharond) - their northern cousins vanished or ruined (for now?). All sorts of opportunities exist. Saints need to help those less fortunate, the magister will need to help out as well after the catastrophy, and likely their companions will have to follow along.
So there you have it: two ideas to help your epic campaign: planar field trip meeting class leveled fiends, etc, and an epic disaster and later recovery program. Have fun!
