I just started playing the challenge and it's great!
And it's not like things like that are only seen in MMOs or mobile games, more and more single player games do similar time limited community challenges. You can nowadays find them in X-com and even Euro Truck Simulator
The only thing I was afraid of, was that they would require a very "gamey", hyper-optimized approach. I'm rather a role player that likes to do immersive stuff, that most of the time isn't very optimal when it comes to "winning" or map paining, but I was able to do silver without much problem by just playing the game like I like, great!
I'm just curious what the plan for the other challenges is. I have most of the DLC and I would LOVE for Paradox to add challenges in less popular places and cultures, that I haven't tried out before. But at the same time this would alienate people that don't have the DLC and make them feel like they are forced into buying additional stuff

It will get kind of boring if those will be only European Catholic rulers. No idea how to solve it :/ Maybe two rulers, one vanilla, and one exotic DLC one?
Dear developers, have you considered it?