One way is to think what other purposes could we add to those dungeons....

Maybe dungeon levequests? for xp (for the lowbies) and GC seals (to entice the players at max rank)

Connecting dungeons together. Clear Dungeon A to get a key item for use in Dungeon B. Basically force players back into the earlier Dungeon or there is little point in doing/or its not possible to go into Dungeon B.

Make the world more unique/diverse. You only get coerls in Dungeon A. The only other way to get coerl hide is to buy them from an npc. Entice people to farm in those Dungeons.

Another suggestion is a type of Hunt system possibly based on a type of levequest. Clear the 10 rank1-10 NMs in Dungeon A to unlock the 10 rank 11-20 NMs in Dungeon B. And so on. Then at endgame you have 10 rank 60 HNMs in Dungeon Z.

Or perhaps, something more simpler, like clear Totorak to get a disposable unisversal key item that lets you enter a variety of BCNMs rank 30,40,50