Artifact helm of telepathy, neutral aligned, that either enhances or blocks telepathy level-wide based on whether you have it from some source other than the helm (or alternatively, it’s not a helm of telepathy, and just bases things on whether you are telepathic), and grants a Charisma-based bonus to your armor class.

If you do have telepathy, all other monsters get telepathy too and you can sense them, and they will be affected by telepathy-targeting effects, including yours if you have any psionic abilities.

If you don’t have telepathy, all other monsters will be treated as if they are not telepathic, even if they are; they will also be blocked from using any mind blasts or other psychic powers.

Eating a wraith’s brain when polymorphed into a mind flayer should give you some experience points.

When a mind flayer removes the grease from your helm, its round immediately ends because it has to clean off its tentacles before attacking again.

Mind flayers (or possibly only master ones) can use telekinesis to drag you a space or two closer to them so they can eat your brain.

If you eat a lizard’s brain while turning to stone, it cures your stoning.

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).

When polymorphed into a mind flayer (or playing an illithid in variants that have it as a race), you gain additional nutrition when you eat the brain of a spellcasting or telepathic monster.

Assuming the monster and player spell lists are unified so that monsters cast player spells: When a mind flayer sucks your brain and causes you to forget a spell, it may gain the ability to cast that spell.

Reduce the impact of a single master mind flayer attack, so that they don’t just spawn out of nowhere (e.g. via harassment) and one- or two-hit kill a character with low Intelligence.