This is a bit of a post, so bare with
Heres my suggestion, feel free to add to it, to completely overhaul M+ , numbers used below is just suggestive for the example.
1 - Remove the RNG of the key and key swapping at the end of the dungeon, Instead, Add a currency that you obtain from completing keys that can be used to buy new keys.
You can use the “Average Rio Score for dungeon level” to determine the cost of a key for example. So if you have timed all 7s and your score is 2200 as an example. Then use the amounts like
+7 = 2100 Currency
+8 = 2200 Currency
+9 = 2300 Currency
Maybe even add in a tweak here where you can spend a bit more currency, to buy some more time on the key.
+7 = Standard key = 2100
+7 = Key with +1 minute to timer = 2150
+7 = Key with +2 minute to timer = 2200
but as a punisher, by taking a key that allows extra time to complete, you get a reduced score for finishing compared to timing a key with no extra time added and by using a key with added time, completing all dungeons using the extra time, should not allow you to gain score high enough to get into a key beyond +2 of your current cap. (as stated the whole number thing would need fine tuning)
2 - The Cap on this currency, is tied to your RIO so you cannot spam +4s to get enough currency to buy a +12 key. When you time your Key, its destroyed, But you gain back the full amount of the currency used to purchase it. Allowing you to buy another key (of your choice) If you complete your key but you dont time it, then you gain back 50% of the currency used to buy the key. So you need to do someone elses key, to gain back the lost currency to buy a new key.
Keys are not destroyed or downgrade when a key is full on failed/disbanded.
Maybe even remove the Unique factor of the key, so if you have a +10 GB key , you can still do other dungeons to get enough currency to buy a 2nd +10 key. All keys are removed at the end of the week as normal.
So by completing more dungeons, you get a better RIO and therefore you get a bigger cap on the currency so you can afford to buy a bigger key or more keys!
Why do I suggest this…
The main issues I see with the system, is it being a challenge, and if people are doing your key, then they have no incentive to stay, I find people instantly quit after the first wipe, or they get “Disconnected” and never come back.
Because of this, we see the “Meta spec only” dungeons where everyone only wants those youtube specs! they rather sit in queue for 20 minutes for a Resto sham, than try another healer in fear of lowering their key. Changing to this system would allow you to try all different comps, if you cant do it and disband you still have your key.
There are generally 2 types of dungeons, the Completion for some gear/crests and then the Beat timer, and these people are the worst in my opinion… they will leave your key in a heart beat if they think its not going well. I remember a few weeks back my push group was missing a tank, so we put up our +10 as “weekly completion - no leavers” … healer accidently made a mistake on 1st boss, he died and we wiped, and without saying a word, the tank just left the key, it was our first death and both dps at the time was like 1.5mil overall, The key was going well, but it was scuffed and we missed out on the +10 vault loot cuz a guy left even though we posted it as we did.
The biggest issue with this, is that you dont really get to practice overcoming your mistake, without reducing your key down and down. Say for example you attempt your first +12, something goes wrong, and Boom, you arent going to time someone instant leaves. Now you can go again at a +11, but you might have timed the +11 easily enough and you dont need to adapt tactics for it, like you do for the +12. But now you have to either try get your team into another +12 of the same dungeon, or you end up somewhere completely different with a new team not having a chance to learn how to overcome.
There is my 2 cents on the system, I think something like this would open up the variety of specs that get invites as you would be more willing to take a chance on specs to see how they perform etc and it allows you to target specific challenges to overcome the key to progress.
Those that arent too concerned with pushing high keys, can opt to buy more time in a key, to get the full crest reward etc if thats their focus instead.