I've got a few doozies. Brace yourselves.
(1) The players join an international coalition lead by a mysterious and charismatic benefactor to oust a brutal expansionist seafaring empire from their home continent. They distinguish themselves in a major battle, earning the favor of the coalition's leader, who showers them with treasures including, wait for it, Magic Rings.
I want to see if I can get them to Numenor before they realize that they are Nazgul. The players will then spend the next few thousand years trying to sabotage Sauron's plans without letting him or the other, much more loyal Nazgul catch on. For so long as no one claims The One Ring, they keep their free will.
(2) A Wendigo, a powerful Oni, a Rakshasa prince, a Serpentfolk Lich, and a Pit Fiend have joined forces. The PCs are ridiculously outclassed. But, thanks to an ancient and obscure tradition dug up by their divine sponsor, there is one way their plans can be stopped. By beating them in an Olympics-style team-sports tournament!
The tournament should culminate in Basketball On The Moon (so I can play the Space Jam theme and make the Serpentfolk Lich do a slam dunk). An Aeon and a Yamaraj psychopomp will referee to prevent cheating and ensure both teams compete on equal footing.
(3) The party is comprised entirely of bards. They are invited/coerced to represent the Prime Material in an interplanar Battle of the Bands. Possible rivals may include an orchestra from Axis, a boyband from Hell, rappers from Limbo, and a power metal band from Elysium (because where ELSE would it come from!?).
.........I'm not sorry.