I am considering attempting to run one of my White Whale campaigns again-- the Planescape/Star*Drive crossover. Players would control two PCs in two "campaigns" running concurrently as the settings slowly merged.

Planescape was one of the more popular settings for 2e AD&D. Set (mostly) in the Outer Planes, Planescape broke the mold of traditional medieval/dungeon fantasy and presented a world of infinite possibilities where reality was shaped by belief.

Star*Drive was the default setting for TSR's Alternity RPG. It's set at the turn of the 26th Century, thirty years after the Second Galactic War and shortly after the war-torn Stellar Nations have re-established contact with the isolated colonies in the Verge. There's another storm brewing over the horizon, though, in the form of the Externals-- aliens from the neighboring spiral arm of our galaxy.

Due to the nature of the campaign, I am thinking to run it via play-by-post.