This collab could be made like this:
Right now on hexquisite, you only see a part of the other hexes, this way you do almost anything you want, while having some guide to make sure the hexes fit with each other.
An improved idea could would work like this:
Everyone select the hexes they will do, they can select more than one, but the hexes they select can't be adjacent to the ones they selected.
They do those hex tiles entirely by themselvess doing anything they want (that follow theme rules and pallete rules) without posting the wip on the site.
After everyone make their tiles an image is made with all those tiles.
Now everyone must redo ENTIRELY their hex tile, doing whatever the hell they want (also following theme rules and pallete rules).
But when redoing their tiles they must MAKE REALLY SURE it fit with the adjacent tiles.
PS:This new hex tile they do can be something totally different from the old tile they made.
An image collab of the part 2 tiles is made and released.
The tiles made on part 1 are just something needed to make this project work.
So with this idea you will have freedom to do whateaver the hell you want, while still being influencied by others just enought to make the collab image works.