The number of turns you can remember a spell is directly dependent on your Intelligence, and possibly Wisdom (at the time of reading the book). The default 20000 turns is a baseline for maybe 12 Int and 12 Wis; more of those means you get more turns when you learn the spell or refresh your memory, and less of those means you get fewer turns.

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).

Eating any kind of fish may increase your Intelligence, because of the popular wisdom that fish is “brain food”.

A new object or, more likely, an artifact which is a ring of enlightenment. Wearing it increases Intelligence and Wisdom by 1. Additionally, when you perform the #attributes command, you are shown your fire, cold, shock, poison, and sleep resistance statuses. Possibly it can be invoked for full enlightenment.

The blessed effect of the scroll of amnesia allows you to choose the skills (and spells?) you want to forget, which allows you to reskill more efficiently.

Possibly only do this if your Intelligence is sufficiently high.

When polymorphed into a form that has automatic bite or other melee-touching attack, if you are about to attack a cockatrice, make it subject to an Intelligence saving throw; if you pass this you avoid attacking the monster with your body.

Remove Wizards’ exclusive access to reduced-hunger and hungerless casting. Instead, tie spell hunger to how experienced you are with the spell; this could either track how many times you’ve cast that specific spell, or use your skill in the spell’s school as a proxy for that, or both. Intelligence could remain a factor, but the important thing is that non-wizard casters can also enjoy the benefits. (Note that Wizards, what with casting spells a lot more than other roles and having high spell skill caps, will still reap a larger benefit from this.)

Remove the player’s Intelligence as a factor in determining the outcome of a foocubus interaction. Instead, use Intelligence (or possibly Wisdom) rather than Charisma as the determining factor in whether they are able to say no to having a piece of armor removed.

A helm of brilliance no longer boosts Wisdom. It only affects Intelligence.

Intrinsic telepathy only works in a radius equal to your Int (extrinsic sources aren’t changed). Cooperative telepathy, if implemented, works in the same radius if intrinsic, full-level if extrinsic.

If your Intelligence is 6 or lower, reading a scroll may give its confused effect even when you are not confused.

Markers no longer have charges; instead they drain Intelligence points.

Intelligence can be abused and exercised, but unlike other stats, exercise will never bring it above its base value (or the value you have raised it to through gain ability).

Sources of abuse: mind flayer brain eating, zombie attacks. Sources of exercise: reading spellbooks, writing spellbooks, writing scrolls, learning a new spell, getting a major Oracle consultation for the first time, possibly some of the uncommon, non-farmable ways stolen from the things that exercise Wisdom.

Your success chance of writing an unknown scroll or spellbook should be based on Intelligence, not Luck.