Occasionally, if a Mines filler level generates where there is a sufficiently large space of empty stone, a little separate walled-off area may generate disconnected from the main map, containing a few gnomes and dwarves, and some more gems and gold, possibly in a chest. This area won’t get the stairs generated in it, and should probably contain a hole so that a randomly teleporting player won’t get stuck there (other types of escape items could get picked up by the gnomes and dwarves).

Monster generation in the Mines automatically selects a G monster on 1/6 or 1/8 of all spawns. It’s odd that the branch’s lawful bias means that dwarves tend to appear over time, but not gnomes.

On the level containing the stairs to the Gnomish Mines, either always or often generate one gnome in a random place.

The Mines have a higher rate of gold and gems embedded in rock than the other branches.

Instead of being completely lit or unlit, Mines levels can generate with large contiguous areas of lit and unlit floor.

Upper Mines levels’ stairs are biased to generate relatively close to each other.

Add shallow water to the Gnomish Mines (not GruntHack’s deep water).