So, here's the premise for my Spelljammer adventure that I'm definitely not going to run:
The characters are each the most famous hero of their respective worlds - they are created as 15th level characters (and can be basically anyone or anything). The players are each encouraged to come up with their own legend, with the one caveat that they have absolutely no knowledge of one another.
Some time after their deaths, they find themselves gathered together in a dark and dismal temple, where they are approached by the local priest, who presents them with their not-at-all-railroaded quest: the priest's patron wants to know the secret hidden in "The Nested Sphere", and has provided a suitable Spelljammer vessel for their use.
As incentive to drive them, their patron has erased their legends from history. If they refuse to help, they will forever be forgotten. If they do as asked, their legends will be restored. Naturally, for legendary heroes the worst possible fate is to be forever forgotten.
The Nested Sphere is an unborn Crystal Sphere, and as such is protected by the Ban of the Unborn, which states that "deities are prohibited from influence, contact, or dealings with preincarnate souls in any fashion whatsoever." As such, their patron can tell them nothing about it. But a broker on the Rock of Bral has something that might be of interest.
From there we go on a tour of the Spheres (this being a classic Spelljammer campaign, not the newfangled abomination). So off to a variety of worlds we go...
Eventually we head to The Spelljammer, and then off to the Nested Sphere, wherein lies a well-guarded library in which a hidden tome holds the secret by which a lesser divinity can ascend to the rank of Overgod. Naturally, Vecna would be most interested in this...
And then comes the showdown - face off against the God of Secrets and suffer being forgotten, or give over the tome, restore your legends... and doom the future?