The challenge modes were fun, interesting takes on the dungeons, normalizing iLvl and adding timers. Iterating gets you the modern M+ system. But… the keys.
How many problems are caused by the key system? Looking through the forums finds almost no positives on having keys that 1) are RNG to get, b) are RNG to get a particular dungeon and level, and III) can be depleted by ONE PERSON leaving a group or ROT because you have bad luck making a group. LFG is going to be hit or miss and is rife with pay for boosting groups.
I propose an overhaul building on the tier unlock system:
ANYONE who has heroic dungeon level gear can run M+ 0 - 5 baseline, meaning you run straight to 5 if you want. Fail badly, maybe a 2, buddy. Then hit that 5 when you are ready.
M+ 0 - base heroic, bumped slightly, with a timer and bonus chests to find - Want to speed run? Timer. But… have extra chests randomly throughout making a full clear attractive too.
M+ 1-5 - affix 1 for the week. Levels and difficulty scale appropriately. Clear +5 and levels 6-10 unlock.
M+ 6 is just 5 with a second affix, then scales appropriately.
If you cannot quite pull off that 10, just run the 9 or 8 until you can.
Fail the timed run? Go for the clear and not feel like you wasted time, and do not lose the run with a rotted key because the tank rage quit for healer not being able to out heal you eating three Tyrannical slams and an orb KEVIN. coughs where was i…?
Give the clear gear and the racers their mount or title for gold timing.
But get rid of the keys. They are a barrier to entry for new and returning, they are a weapon for the toxic, discourages learning and experimenting, and it just feels bad to have one rot if you have trouble making a group this week.
BONUS THOUGHT:
Here is a thought on challenges: 6-10 is the same level scale as 1-5 but with a second affix, then 11-15 same with a THIRD, and so on. Make them challenges with managing 5 affixes. Or even add them as you go through a dungeon or … raid. 5 man version of the raid with affixes being added as each boss is downed.