I recall the proving ground in WoD that while slightly tedious really did improve the queue experience for most players but what if we applied something similar to all group content?
What if to raid mythic you had to complete a mage tower with a preset character? Then an easier version for heroic, normal, lfr?
It would help show groups that you have conquered something roughly at the level of play needed to succeed in group content and might really help new players break out of the raider io bottleneck the game currently has.
I admit I can't really see how it would work out for mythic plus... maybe a new challenge every 5 keystone levels?