We can already see what the highest level of the dungeon people have completed. Change this to “Level Available” instead, and just like keys, you can +3 a +5 key and your available level would jump from a +5 to a +8. It wouldn’t work much different than it does today, it’s just keeping people from trying to join keys higher than they’ve proven they should have access to.
Imagine also having parameters able to be set alongside the level of dungeon when choosing Mythic+, to set what level, such that groups forming for a +5 would not appear in search lists for players who only have +3 available. It would significantly cut down on applicants from undergeared people looking for a carry.
This can easily be addressed by treating it just as how keys are currently treated the first week of a new season. Players don’t start back with a +2 key, they get a key that’s what, 5 levels lower than the previous season? No difference here.
I can respect disagreement here. Maybe just go the other way, and allow access to the highest available in the party. But add a parameter to require a certain level available, restricted to no higher than the group leader’s level.