I'm actually approaching this challenge myself, because in addition to running my long-running Ptolus games, which take a lot of mental time and energy and now move at a George R. R. Martin pace, I want to have a beer-and-pretzels choice always on the table, probably using
Shadowdark.
For me, that means building a
West Marches-style game where whoever shows up each week is fine, with the expectation that other player characters are sleeping off a hangover at the inn or puttering around in their wizard's tower or whatever.
I know I want it to be a standard OSR vibe game, although with the Shadowdark world assumptions (playable goblins, for instance), but I also want it to have a sword & sorcery past, so that I can bring run adventures like that when needed and test a custom sword & sorcery class, etc. So I know what happened 5,000 years ago to the sword & sorcery setting and how it got replaced with a world that now includes elves, dwarves and halflings. (This also has the benefit of having antediluvian human ruins and hidden civilizations around, along with having ancient and depraved humans, rather than drow or the other usual suspects, be the big villains of the setting.)
My plan now is, in a moment when I've got some free time, is to drop a fortified town into the middle of a hex map and then sketch in a wilderness province around it where there's only one real safe home base, but a number of dungeons and ruins, wilderness of various sorts, etc., to explore.
I was tempted to make this a nautical adventure, with lots of islands, but two of my players did a lot of that with me during the pandemic, and I think primarily focusing on a classic Borderlands area will work for me. If there are fun one-shot islands (one of the Shadowdark starter adventures takes place on a sinking island and one of the zines is a Viking-flavored nautical adventure), we can just hand wave the players over there, the way that Conan popped all around the map in his stories until post-Howard writers decided to figure out his timeline better.
Right now, I just know the names of a few gods (generated more or less on the fly for my recent Grizzled Adventurers games), which I've kept in a notebook, and the names of a few locations, like Echo Lake. The rest I will leave very loosey goosey and construct as I go.
Most of the actual work will be creating a home base, the immediate area around it, and a few interesting places for people to go look at on the map. Then I'll provide that to my prospective players (via Discord and our private Facebook group) and let them know when they want to check something out and we'll arrange Shadowdark games that way.
Then I go back to the more labor-intensive hand-crafting of the separate 5E content, which we'll do as it gets done.