What I would also really love is changing the rewards up a bit, especially for the harder challenges. The challenges demand so much stuff only to be exchanged for shulker boxes or some diamonds.
For example: The zoo challenge itself would give you 32x diamonds and 2 shulker boxes for getting all those mobs to spawn.
It's been a month since I tried, so far no luck at spawns. For a challenge which feels near impossible, I would love the rewards to be useful stuff instead rather than some diamonds (only use for diamonds is for raising is level and you can farm more emeralds during the time you gather the mobs required to complete the challenge) and shulker boxes (Shulker shells can be farmed from panda's mob farm). The rewards could just be stuff that could indirectly help with other challenges.
(I got a sneak peek on the elite level challenge, which requires a hostile mob farm with mobs which are not possible to spawn, maybe the rewards could lead upto getting eggs for those so they are doable). At the moment, challenges are just too hard to accomplish and the rewards just are not worth it for us to try to complete it.