Currently, the matchmaking system for the dungeon, I assume, involves randomly matching 5 players in queue and putting them in a party.
The inherent problem with this is that these parties tend to lack a healer and a tank. Which needlessly decreases the survivability of the party in the dungeon, as well as the efficiency of the party.
I understand that if the presence of a healer and a tank was made a requirement for each matchmaking party, the queue time would be horrendous.
As such, I would like to suggest another approach to the problem, something another successful mmorpg has done. This includes having separate entry limits for each dungeon, and having high lvl characters get a damage penalty that is inversely proportional to the amount of levels they are above the recommended level for the dungeon.
While I understand this is not the best method for this game due to its unique class and rank system, this methods provides the benefit of keeping low level dungeons lively and increasing the population of healers and tanks in the matchmaking queue. This subsequently allows you to implement a system whereby healers and tanks can be included as a requirement for each party.
Not to mention this potentially prevents new players from entering the game, only to find out they can’t do the dungeon due to all other players being high leveled, resulting in no one queuing for the low level dungeons.