Add two new elemental planes, those of Positive Energy and Negative Energy.

The Plane of Positive Energy:

• The full level is lit.
• Has lots of yellow lights, trees, and whatever plant-based monsters exist in the game.
• Everything is treated as if it has regeneration. The player can regenerate current HP past its maximum, but if this gets too far past the maximum, they instantly die from being unable to contain the energy (there are warning messages to this effect as you approach this point).
• Items in the player’s and monsters’ inventories can randomly get blessed, enchanted, or charged - even if this runs the risk of overenchanting and destroying them.
• Eating any food restores HP as well as nutrition.
• All exercisable attributes are exercised at regular intervals.
• All sleep effects are negated.

The Plane of Negative Energy:

• The full level is dark.
• Contains lots of undead.
• Everything constantly loses hit points. If you have regeneration, it either nullifies or only partly nullifies this effect.
• Items in the player’s and monsters’ inventories can randomly get cursed or lose enchantment or charges (going negative on things which are capable of being negative).
• You lose several times as much nutrition per turn as normal.
• All abusable attributes are abused at regular intervals.

In the context of restoring the tension and strategizing created by encountering a mind flayer, which used to be a lot higher when their amnesia caused forgetting maps and discoveries:

The psychic blast of a mind flayer may make you hallucinate, to create short-term tension, and possibly create long-term strategic tension by counting as Intelligence abuse.

However, since mind blasts are a lot less avoidable and harder to strategize around than getting into melee with a mind flayer, it may be better to add these as additional things they can do in melee (but probably not Intelligence abuse, since they already directly drain Intelligence points).

The potion of orcish war juice should be at least situationally useful for heroes of any race. One way of doing this is to make it more in line with the portrayal of a similar substance in The Lord Of The Rings: have it give useful short term buffs to anyone (minor healing, cure wounded legs, one point of increased strength) with long-term penalties such as a boatload of abuse. Meanwhile, the potion remains a good thing for orcish characters to drink: they get much more healing, bigger stat boosts, and no long-term penalties (though there may be short-term penalties such as mental stat reductions).

Wearing a blessed ring of sustain ability lets you exercise attributes, but not abuse them. A cursed one lets you abuse attributes, but not exercise them. It continues to block all other sources of ability score change. If wearing two rings of sustain ability, determine what happens by averaging the beatitude of the two.

Being satiated abuses wisdom for Samurai (at the same rate it does for Monks currently), and gives Monks slow alignment penalties instead of abusing wisdom.

Consorting with foocubi abuses Wisdom.

When you throw ammo without the proper launcher, the game makes it clear that you’re doing it wrong, e.g. “You clumsily throw the arrow”. This also abuses Wisdom.

Exercise or abuse Charisma as a result of consorting with foocubi.

Vaporizing gear by overenchanting it abuses Wisdom.

Gradually, over time, you lose points of Charisma due to accumulating dirt and scars and generally looking worse. This can be fixed with the usual restore ability methods, by entering water (possibly sitting on fountains should do this), by getting hit by a rust trap (this only restores one point at a time), or by #wiping with a towel (which may have limited effectiveness).

Condensing all of NetHack’s poison mechanics into one unified mechanic:

• Dexterity and Constitution poisons are removed. Quasits (the only AD_DRDX attack) might still directly harm Dex, but it’s no longer flavored as poison. Rabid rats (the only AD_DRCO attack) don’t cause poison, they cause disease, which hits Constitution. Poison will now only affect Strength.
• Poison is now implemented as a timing-out trinsic. The amount of Str loss is a direct function of how much timeout is left, so Str will gradually recover as the poison goes away.
• Passing Con saving throws could give small bonuses to decreasing the timeout faster than normal.
• While poisoned, HP regeneration is either impeded or stopped entirely, or else the player’s HP is damaged every turn, or every several turns.
• It could also heavily abuse Str while it lasted instead of being directly tied to Str.