The Tavern

I have a feeling that as you progress in the game, the monster density raises while individual monsters get less threatening. I feel that this leads to bad gameplay.

This is especially true for Depths - there are no enemies that are dangerous alone, but there are so many monsters that it is really tiring to kill them all. Zot at least has orb of fire and ancient liches which can be dangerous alone to some characters, and Vaults has the door closing monster to block escape.

I think that Depths would be much more enjoyable if you cut down the number of enemies to half but increase the difficulty of individual monsters. I know that this could be big work especially as many enemies are generated by vaults, but if there is a decision that it would be good than slowly it could be possible to go towards this goal.

The idea sounds quite alright to me. I would have said that there have been some steps in that direction: caustic shrikes, juggernauts.

There's something of a power creep problem to this: instead of making monsters harder, we could weaken the player characters (although that tends to get less liked). Myself, I reach Depths rarely enough to be challenged. I do agree that the hordes can be tedious.

Along those lines, culling some weaklings from depths would be good too. I'm thinking mostly of plain Ogres. In both depths and vaults they are almost never threatening. Even when they get buffed somehow.

And now one has to wonder why iron giants had to be removed from Depths.

The biggest threat in depths is in fact being bogged down. Luring one by one is so tedious that I don't do it and the game is harder yet more fun because of it.