Sorry if this is in the wrong place, but Behind the Scenes seemed like it's only for games being run here.
Planescape/Star•Drive: Renaissance is one of my White Whales-- huge, ambitious campaigns that I've been trying to make work for years. The absence of a common system, and then the absence of mutual fans, has kept this one out of my grasp for going on twenty years.
Now, I've got two systems I can run this in-- I can use Cortex Prime, which I think is better suited for Planescape than D&D, or I've figured out a way to unify D&D 3.X with d20 Modern in a way that can support both sides of this campaign.
Both sides, because it starts out as two separate campaigns: players control a PC in each setting, and the games run parallel as their respective settings slowly intersect and then merge. They have to solve the mystery of why the Star•Drive universe has been isolated from the multiverse for millennia, and why it's slowly reintegrating itself, to prevent devastation on a Multiversal scale.
If this sounds good to you, check out the rest of the details at: https://www.myth-weavers.com/forumdisplay.php?f=43143&tg=82003