Make the prize for Sokoban a guaranteed amulet of reflection, and guarantee a bag of holding to generate elsewhere. It could be in any of:

• On Mines’ End, not replacing the luckstone but along it.
• Somewhere below Minetown, not necessarily at Mines’ End.
• In a vault in the Dungeons of Doom, not in the first few levels.
• Somewhere at or above the depth of Mines’ End, which could be in any branch.

However, guaranteeing the existence of a reward item like the amulet of reflection risks funneling players into a single standard strategy.

Every floor of the Gnomish Mines should have a small chance of generating a “mining accident”: a smallish area containing scattered rocks and boulders, at least 1 dwarf corpse with regular starting inventory, and 50% chance each of a digging tool and a potion of booze. Ideally, this will be near or against a wall rather than out in the middle of an open area.

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).