Utterly Mad

Settings I personally like:

Post Apocalyptic - Nuclear, zombies, robots out of control, aliens, whatever. It always makes a good backdrop, especially for this type of project, simply because I can write a story about Fred's enclave in north America, and someone else can choose to go a completely different direction and talk about the re-emergence of the Mongol Empire in Asia, and it can all exist in the same universe while still being somewhat plausible. Plus, technology is easy to figure out because everything could be low tech, today tech, or near future tech, once again all in the same universe.

Post apocalyptic doesn't even have to mean nukes and zombies. We could have a world that exists right after the fall of the Western Roman Empire or something - I'm meaning a setting where major changes have just taken place, or are taking place in the world.

Alternate History - Wild West, WWI/WWII, Napoleanic Wars that never really ended, North Korea actually won, the Saxons beat the Normans and England never became the world-dominating empire it did because of this, whatever. Same logic as above, people can write about radically different things and still have plausibility.

A Grimdark Far Future - Same logic. Look how much completely different shit comes out of 40K writers. If the universe is a literal universe, then pretty much anything can be plausible.

A completely different idea could be to not actually build a setting. Use the real world, but write about some secret aspect of it, kinda like SCP or similar. We could build on this after writing the groundwork, and have things kind of evolve as we write them.

Honestly, I think we're all pretty creative and can work with anything, and I'm pretty good going with a strict cannon for us to adhere to, or making it up on the fly. About the only thing I'm not personally interested in would be high fantasy. I just don't dig magic and dragons and stuff, and don't know what I'd really have to contribute if we go that direction. No problem if magic is in a more "real" setting  (like Larry Corea's Hard Magic series, if anyone's read that), but I just don't have a ton of interest in writing Tolkien-esque stuff. That's just me, though.

I think timeline and major events should wait until we figure out a setting though. I mean, if it's alternate history, then a major event could be Tesla's invention of a death ray that causes the RL WWI to happen in the 1950s instead of the 1910s. If it's set in the far future, then the same thing could have maybe occurred, but no one would care 20,000 years later, because current events are focused on a war with a human colony that's been separated  from Earth's Colonial Authority for a few thousand years, and who cares what happened way back when?

When building a timeline, it seems like it would make more sense to fill in only a few really defining moments rather than laying out a really clear course of events. Then we can write what we like and not worry about stepping on toes so much.

Anyway, that's my thoughts.