I think from now on I'm just going to use Mystara as my campaign setting. If the players do something it affects the world moving forward. If I play an old school campaign or a fifth edition campaign, doesn't matter. A few things have crossed my mind latetly:
Old School Campaign:
After reading the back of The grand duchy of karameikos gazetteer, There's lots of little adventure ideas. My first thought would be for the players to start in Luln, and investigate the old castle (which would be the sample dungeon from basic). From there they can get hired by Gnomish caravan to bring goods to Specularum. During the trip a major rain storm will hit, causing the caravan to get stuck in the mud, falling over, spilling out the contents of one of the wagons. There the PCs will uncover the map & letter regarding Isle Of Dread. Once in Specularum I think a perfect adventure would be "Veiled Society". After that the PCs will hopefully have enough money to make the trip to the Isle of Dread. (And be of decent level). Since Specularum isn't that detailed in the gazeetteer, I think using specific bits & pieces of Blacksand should do the trick. There's a lot of good information for sections of a city in the book. Should be easy to use that. At some point maybe "A nights dark terror" or "castle amber" might make an appearance.
Fifth Edition Campaign:
I've been reading a pile of Al-Qadim stuff latetly. The land of fate is a really good book. In Mystara I could pretty much set everything in the Emirate of Ylaruam, in the city of Tel Akbir. I really dug "Golden Voyages" and a "Dozen and one adventures". I think they would be relatively easy to convert to fifth edition. At least bullet points, rather than the adventures (In the case of a dozen and one adventures). There are a lot more combat opportunities in the 2nd edition adventures, and because combat in fifth takes for friggin ever I'd probably trim it down a bit.
With Golden Voyages I think I would redraw the ocean map near the coast of Tel Akbir. For the most part I think I'm going to sit down and do a bit of detailing for the city of Tel Akbir & Dozen and one adventures. If I can get 2 or 3 adventures re-written & converted for Fifth that should give me enough ammo if I ever have to run a one shot campaign. I guess I'll create a wiki for the players.
Oh and the gods are going to be the immortals. Until the players reach 20th level, then they can replace em.
That's enough rambling for me this morning, cheers!