I want to start by saying that I do like the new challenge system; however, I can see the potential for problems with it moving forward.
First, please make the weekly challenges something achievable during the flight. When only slayer is playable and most of your weekly challenges are for strongholds, it is no longer a challenge but an impossibility.
If not addressed this could become a problem with the final game as well. We all have seen many of our favorite game types become temporary events. It would just be nice to make sure that whatever the random weekly challenges are, they are something that is achievable that week, regardless of your available play time.
Swaps are not the answer, when you randomly change to something else that is not achievable for the time being.