EDIT: Those challenges will be added as a part of the Iron Rancher mode, where you'll have to collect quotas of various plorts across multi-day rounds. Each round will have remixed rules and unique challenges listed here. They will be added with it in a version post-1.0.0.
There will be few different challenges to select, some will barely do anything, while others might drastically change a thing.
The first mod will make chickens seek out other beings like Slimes and you, and try to kill them.
2. Night Tarr! (Bad Moon Arises!)
At night, there will be a chance that one of the natural spawns of slimes will be replaced with the Tarr.
3. The Gambler (Plort Shuffler)
Ślimes now have a chance to produce a random plort instead of its normal one.
4. Gobs of Globs
It's pretty straightforward. Slimes will spawn few times as much they do normally.
5. Slime Hunger
Slimes will get hungry much faster. It will be a lot harder to keep slimes in corrals.
6. Unstable Plorts
What would this do? I'm not sure. Possibly the Plort Market prices will crazily fall and rise each day.
Speculate in the comments!