I've been thinking, since this is the home of planescape, this mean we can create new 'canon,' right?
Well, there are a couple of things i can see that could be tackled by a planewalker adventure.
1: The faction's return to sigil
2: The continuation of the RRakkma, but with a twist
3: The Far Realms (there have been hints to something big coming all over the place, its about time something actually happened)
4: The Celestials start taking a more active roll in the fight of good vs evil
5: Another Yugoloth plot to rule the world
6: Some combination of the above
Maybe this plan will fall apart, maybe this is one of those ideas that should be kept to one's self. I dunno.
This is one of the things being planned for the PSCS actually - Right now we're working on slamming out the rest of the CS book, and then moving onwards with modules and the like. In the meantime, if you want, Kyto's Hooks actually is accepting modules and larger adventures like what you're looking at - why not put one together and submit it there?
That way you'll have some practice at writing them and the PSCS writers can take a gander at the works on the site and use them for ideas or tap writers that seem to match up well.