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.

Eating the brains of a spellcasting monster should restore some magic energy, and perhaps even have a chance of learning a spell from its brain. (This latter part would work better in variants such as FIQHack where monsters have a list of player spells).

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.

If you are eating someone’s brains when polymorphed into a mind flayer, and you are very satiated to the point where you aren’t getting the full amount of nutrition from it, you do less damage with the attack because you aren’t eating as much of their brain.

Some polyform (perhaps mind flayer) that doesn’t forget spells over time while you remain polymorphed into it.

If a mind flayer eats your brains while you are confused, it becomes confused.

Mind flayers have to grapple your head before they can successfully suck your brain (like in D&D), forcing them to spend a turn doing this. If you have a greased helm, this gives you at least one extra turn to act since they will not be able to grapple your head until the grease dissolves.

This would probably best be implemented by giving them a sticky grab or bear hug attack and ignoring the existence of their tentacle attack if they aren’t already grappling you (or you are being held by another enemy and thus can’t avoid them). Either way, checking u.ustuck is the key.

Possibly, they need to spend another attack getting your helm off, which will unequip it, though this will get weird if your helm is cursed. A cursed helm shouldn’t offer total protection against them. Possibly, they should be able to remove a cursed helm (like nymphs and foocubi do) despite its curse; this could be handwaved as them using their psionic powers to do it.

In variants that implement zombies eating your brain, it should work on basically this same system, where the zombie needs to hold onto you before it can eat your brain (and which also works that way in D&D).

Eating brains while polyselfed into a mind flayer recovers magic power instead of nutrition.

Mind flayers cannot tentacle you if they are cancelled.

Whenever a player is killed by a mind flayer, they become a thrall in their bones file. In FIQHack, this means they leave an active hostile player monster in the bones with 100% chance instead of 33%.

Mind flayers seek out your pets to eat their brains and turn them into thralls, which makes them effectively a pet of the mind flayer instead.