Credit where it's due, Reddit was pretty good at coming up with novel ideas for social experiments for a few years (which also drove engagement, not coincidentally).

If I were to come up with Place-but-not, for the fediverse and Lemmy specifically, I would do it like this:

The canvas would start very small and divided into plots, with each plot "owned" by a user. The user who owns a plot can determine the pallete to be used in their plot, can whitelist / blacklist other users on their plot, and has a reduced cooldown on placing pixels inside their plot. They are the admin of their plot, basically, which is to mimic an instance.

When every pixel of the canvas has been covered at least once or a certain amount of time has elapsed, it would expand with new auto-generated plots randomly assigned to users from among those who have placed a pixel. Plots could be regular squares or other irregular shapes. The most inactive plots could be blanked and reassigned to a new owner after a time.

In this way, users would have to work together to make bigger art on the canvas or seek out a spot willing to cooperate with their art. You'd see alliances of plots, users making art around an uncooperative plot, hostile plots get ganged up on, hands-off plot owners allowing anything on their plot, and all sorts of shit like that that makes social experiments like this interesting. You'd likely still need top-tier admin intervention to remove hate symbols and the like.