Challenges are good but its sorta like "Here is a challenge' and folks rush off to their labs and start cooking up stuff.

How about tag team challenges?
4 artists go into the steel cage and 2 walk or crawl out!
Well, not that intense but could be more fun than being hit with a chair.

Great modeller but not so good with the scene layout, find a team mate thats good would scene composition. Have seen other sites when artists collab and the end result can look better than what the 2 artists do on their own. Can be a good learning experiance working with another person.

Maybe at the end of the year a great collab battle royal!
4 on 4 collab battles!
Let bounce the metaballs of art!

Some time ago, I don't remember which gallery, they had a group collab project.
We had to do a room with a toybox, somebody started the scene and people added to it.
At the end we had an interesting scene.
This is a way to get a community, projects that people do together.
We keep the regular challenges too, that will not change.