Op here who did the old monthly challenges before stopping. There was little to no traffic. One challenge even got 404ed.
The way I did it was that there were 3 themes to chose from to get everyone involved. One would be a specific theme and the other two were abstract ones (mostly). Mind you that was after trying a couple different ways of structuring the challenge.
Here are the older ones I did for reference (the other challenges from the former OP are on yafuu aswell):
>January:https://archive.nyafuu.org/p/thread/3600575/#3600575>February:https://archive.nyafuu.org/p/thread/3611829/#3611829>March:https://archive.nyafuu.org/p/thread/3625000/#3625000>April:https://archive.nyafuu.org/p/thread/3637653/#3637653>May:https://archive.nyafuu.org/p/thread/3658152/#3658152>June:https://archive.nyafuu.org/p/thread/3666348/#3666348 