I've run into plenty of D&D players who are big fans of one campaign setting or another and tend to not pay much (or any) attention to the rest. I grew up enamored with all the early worlds and found something enjoyable within each, never restricting myself to just one or two. But when Planescape arrived in 1994, it utterly blew my mind. Not only were the concepts, philosophy, artwork, and overall production value off the charts, but just the entire idea that non-epic level characters could adventure within the "framework" of this vast multiverse that ties it all together was enthralling to me. For decades I've toyed with the idea of creating some kind of multi-world odyssey; a story that spans several planets and settings, with Planescape as the glue that binds it.
The idea I will present in this thread could be used for a variety of applications, the most obvious being a tabletop campaign. I think it would also make for a great novel or graphic novel line. Even better, a television series, due to its episodic nature of jumping from world to world (think Quantum Leap meets Sliders). Years ago I thought this would make a terrific video game, this was when Dungeons and Dragons Online was an Eberron product only, before they added Forgotten Realms and Ravenloft zones. From what I can gather there is no overarching plotline to link those areas together, you simply click a door or portal and transport to that content. So this idea could still be viable if some studio were keen on making a game designed from the ground up to incorporate the lands of Greyhawk, Dragonlance, Forgotten Realms, Ravenloft, Darksun, as well as planar content, into one cohesive story.
Post #2 will be a recap on the story of Aoskar, probably unnecessary for most on this site, but worth putting out there nonetheless if anyone else were to wander in and give it a read.
Post #3 is where the story starts to move on from what is known in canon and present the framework for this adventure.