Tortles have a Dex-based chance of avoiding getting decapitated by ducking their head back into their shell.

The “Construction Patch”:

• When digging out stone (not necessarily walls) with a pick, rocks may fall on your head with a lowish chance, about 10% per dug square. These deal the least damage if you are wearing a dwarven helm, and a little less than normal if you are wearing some other hard helm. Dwarvish characters have a high chance of dodging the falling debris entirely. Possibly also allow a boulder falling on your head from digging similar to when monsters tunnel.
• You can apply rocks at an adjacent floor square to build a wall. It takes 50 rocks, or 25 if there is a boulder in that square. (If the space is marked as being wall-nondiggable, this either doesn’t work, or flips the space to being diggable.) It takes 1 turn per rock to build it. In the process it creates an immovable object or dungeon feature called “partially-built wall” on that space, which tracks how many rocks are part of it. If you move onto the space, you can dismantle the wall and retrieve the rocks.
• You can apply rocks at an adjacent pool or moat to attempt to fill it. The more rocks, the higher the chance of filling it. This takes only 1 turn.
• Stone to fleshing diggable wall turns it into huge chunks of meat.
• Force bolts from striking wands or the spell scatter rocks off diggable walls they hit, and in roughly 5 shots the wall is destroyed.
• Digging a pit or hole puts a pile of rocks on a random adjacent square.
• When you dig a hole and fall down to the next level, you are usually accompanied by a shower of rocks falling on and around you. Dwarves are more adept at dodging and getting hit by fewer rocks.
• When using a pick to dig a square of natural stone where the four orthogonally adjacent squares are all dug out already, you are asked if you want to carve out a boulder. If you say yes, a boulder is deposited on the new space.

Note that the two boulder-creation methods above run the risk of being exploited for abusable quantities of food if you have stone to flesh available, and the stone-to-flesh-walls method explicitly invokes this.

If you are a centaur (via polymorph or some variants’ centaur race), you can joust things without needing to ride another monster.

Increase the Charisma cap for elves to 21. Any Charisma score over 18 may automatically frighten monsters via being so unnaturally beautiful.

Elvish players start the game with knowledge of one low-level enchantment spell that is not sleep or charm monster. They do not start with a spellbook of that spell, though. (If they get the same spell in a spellbook as a Wizard, it could either choose a different one or just not count.)

Gnomes, being much smaller than other races, hunger significantly less quickly.

Real-life creatures tend to eat an amount of food proportional to their weight, so the hunger rate in NetHack should depend on your race’s (or your polyform’s) base weight. This mainly helps gnomes by cutting their nutrition rate significantly.

Gnomes (or a potential Tinkerer role) can loot statues without destroying them.

Gnomes get permanent detection of gold and gems within a radius 3. Or possibly the radius can scale with XL.

Elven characters can read the runes on runed objects (which presumably don’t say much of interest, except possibly a runed wand will say what it is).

Archeologists (maybe gnomes too) start the game with all valuable and worthless gems (but not gray stones) identified. If this is too powerful, perhaps they only begin the game with all worthless glass identified.

One of the playable races, possibly halfling, should get a permanent +1 to Luck similar to the full moon effects.

Add some incentive for orc characters to prefer orcish shields over elven ones.

Gnomish characters read spellbooks faster than other races.

Elves get low-light vision, which doubles the radius of light sources, rather than infravision. It also gives them vision on dark squares that are orthogonally or diagonally adjacent to a lit square.

Giant characters or characters polymorphed into a giant should have a boosted carrying capacity.

Dwarven characters always passively see all gold on a level, with the possible exception of gold carried by a monster.

Humans have a slight advantage in their chance of taming creatures.

Elves should be able to squeeze between two trees diagonally.

You can loot trees to get less fruit than you might by kicking it, but the chance of getting bees is much reduced. This might fail, in which case you might fall out of the tree and abuse Dexterity or something similar. If you are polymorphed into a Y, it has a 100% chance of success; if a human or elf, probably 80%; if an orc, 40-50%; if a gnome or a dwarf, 1-5%.

Gnomes get a racial bonus using aklys and crossbow. Should probably be implemented by affiliating an item type with a certain race, adding this as a new object attribute, and only specifying it on racial items (elven armor, orcish weapons, the aklys and crossbow, etc.)