Alright, I've added to the Discord convo, but I'll plunk my suggestions here. Now, I've got a bunch of basic outlines for worlds I'm not super-attached to, but maybe some of these help form the basis:
WORLD OF BARSON/THERUS: http://sharkerbob.blogspot.com/2016/01/world-of-barsontherus.html
Barson/Therus is the Earth 250 million years in the future. A race of humanoids with red skin are the dominant sapients, who have a level of tech just about on the level of the Industrial Revolution. There may be some magic, maybe not. There are two other races on the small continent, a race of bird nomads (the Du'Rast) and Rabbit forest dwellers (Lunetha).
It was an RPG setting where players were to explore the weird hidden mysterious of the past on the small continent, and possible war/intrigue stuff on the big continent. Never did anything with it, though.
Kinda dull, honestly, now that I'm looking at it again. Hrm.
ETHYRIUM UNIVERSE: http://sharkerbob.blogspot.com/2016/01/ethyrium-universe.html
A science fantasy setting. Characters are low powered, mostly pulp-style abilities, but its a universe with it's own set of physics, not dissimilar to our own, but based on the super-science energy/element of Ethyrium. All the planets are all planetoids with one or two-environment themes. Tons of potential for variety, and different creatures and mysteries if wanted. As mentioned in the article, it takes cues from the RPG "Slipstream":
However, may be TOO much of a sandbox, and if people just stick to their own made up planets, there won't be much crossover, so it may not even be what the collab is looking for.
WORLD OF FLOTSAM: Don't have a write up on this one yet.
Floatsam is a gas giant with floating continents, alla Skies of Arcadia. While technically existing in a sci-fi space opera world, Flotsam itself is low tech, since most galactic tech gets fried by the planets energy fields. Most civilization is various races and their descendants crashing on the planet or being exiled there so it's lower-tech, probably comparable to, I dunno, 1900 on Earth. It's also a very sort of frontier world in some ways, being really huge. Because the planet is so big (Neptune sized) with continents ranging from Australia-sized to earth sized, there's tons of potential for all sorts of wacky different things.
There is one empire, but it's attempt to conquer the planet just isn't feasible. They have a presence all around the world, but are very ineffectual at maintaining order outside their home continent. That said, they are the only large-scale peacekeeping force that keeps the planet from being a total pirate-world in some areas.
There are only a few rare teleport devices that allow people and things off planet, so there may also be some use for the world as a rare element mine for the galactic society.
However, this world also has the same problem as Ethyrium, in that its probably too much of a sandbox.