I've only ran one campaign that approached epic level, and even then the PCs stopped just after 20th level. There are several fun things you can do at this point that don't require you to buy another product, though none of these answer the question "What happens after 20th level?" Instead they offer story reasons why the PCs don't go past 20th level:
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Heirs & Apprentices: Have the players take on the roles of the PCs' heirs (or apprentices if the PCs don't have children). This is really fun - you have all this rich material to weave into the new characters' backgrounds. For example, if their father was a great swordsman, perhaps the 1st-level mage son has some fencing skills, speaks as a fencer, and is quick to defend his father. He does everything he can to impress his father, but never quite measures up.
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Alternate Form: Have the high-level PCs be shifted into the bodies of commoners or even animals. Perhaps they are cursed, or perhaps they willingly undergoe the transformation as part of an elaborate surveillance project (but nobody told me we couldn't reverse it!). They now advance in racial levels according to their new forms. Admitedly while the "donkey racial class" may not sound glamorous, it could be a fun chance to create a Shrek-like character. Of course, gaining levels as a manticore or even a vampire or demon could be quite interesting, not to mention harrowing. The longer the character lives the more evil deeds they are likely to commit. Can the good they do overshadow the evil? Alternately the PCs may become angelic beings, torn between their mortal natures and divine selves (as in the movie Wings of Desire remade as City of Angels). Can the PCs cut it as celestial agents or will they fall?
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Amnesia: Strip them of everything they own and give them amnesia. Perhaps they are wrongly imprisoned (as in the Count of Monte Cristo) and forget their old abilities in jail. When they finally do escape they must relearn and remake themselves to get revenge on whoever sent them into prison/exile. Alternately, they may be in a violent shipwreck and are found in a fishing village. The amnesiacs PC are raised by the local fishermen until one day the village is attacked and the PCs discover they are much more than they thought before.
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Members of a prestige class: Design a prestige class into which all the PCs will enter, like "Servants of the Elf Queen." This prestige class should account for all types of characters. Of course, once the PCs get all 10 levels of the PrC you are faced with your original question...
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Past Life or Ghost-Possession: The PCs delve into memories of their past lives, or find themselves to be possessed by ghosts. The campaign must focus on discovering the true nature of their past selves/ the ghosts living inside them. Each PC chooses a new character class and multiclasses in it. A major theme of this adventure would be uniting their past and present selves, or coming to terms with their ghosts.
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Light & Shadow Selves: PCs are cleaved in two through powerful magic. One side is their light nature (which the player takes control of). Their other side is their shadow nature, which is played by another player at certain points during the game. Thus each player in the group controls 2 characters - their own, and another player's shadow. Divide the character's levels in half between the light and shadow selves. The goal of the campaign is to find a way to unify the disparate parts. Along the way the PCs may flee from their shadow selves, have to answer for crimes committed by the shadow self, or hunt their shadow self to kill it.
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Curse: Personally, I don't like this option, but if done artfully it could work. The PCs are cursed by someone who has been harmed by their ambition. Should they ever show ambition or pride they risk transforming into an evil beast. One way to run this is to force any PC wishing to advance to take 5 levels in a "cursed" class before advancing. However, each level of the class makes them less and less human & more bestial and savage. Ultimately, the PCs must find the hag/demi-god/demon who cursed them and petition to have the curse lifted.
Cheers!
