I can already think of a bunch of challenges for maps that don't currently have them.
On Castleween, for example, they could make a challenge to win before the castle starts to break (Indestructible). Or Win after the entire castle is broken (Everything is Okay). And obviously on all of the maps there would be the "Win 5 Quick Matches Achievement".
On "Conveyor" they could make a challenge similar to the ice one on "Ichiban" where you would win a round while inside for the conveyor to save yourself (Package has Arrived). They could also make one where you have to win whilst on the treadmill (The Flash).
These are just a few of my ideas for some challenges. I think it would be very beneficial, and would make buying skins a whole lot easier.