The Wizard of Yendor can only use Double Trouble once per reincarnation.

The Wizard can be fooled into stealing a fake Amulet of Yendor if you are wearing it instead of the real one, but this only works once.

Some monster (a boss? The Wizard of Yendor? Imps?) will use custom taunts against you that are taken from checking the logfile for the last three or four ways you died.

This could also be implemented as non-verbal taunts where the monster uses the way you died against you - e.g. if you died to a zombie, they will summon zombies; if you fell into a pit, they will create a pit underneath you, etc.

Make the Wizard of Yendor naturally magic-resistant, so that he cannot be defeated trivially by a death ray, but compensate by removing his Double Trouble ability. The current state of affairs is somewhat like a lottery in whether you will be facing him with magic resistance or get unlucky enough for him to duplicate, leading to a lot of variance in how hard the ascension run is.

If the Wizard of Yendor has magic resistance and is immune to death rays, he will never cast Double Trouble.

There should be more variety in the Wizard’s dialogue after he resurrects or levelports to you, rather than the same old “So thou thought thou couldst kill me fool” line every time.

The Wizard tracks how many times you have blundered into unseen traps, engraved Elbereth, been lifesaved, and had your god fix your HP, and he mixes insults about embarrassing amounts of these into his taunts.

You can genocide the Wizard of Yendor (possibly only once, in his initial incarnation). This will leave the Book dropped in his tower, but since this counts as killing him, harassment begins immediately. When he returns after being genocided, he says “So… thou thought thou couldst genocide me, fool.”

The Wizard will only cast Double Trouble if he has already been killed at least once.

When the Wizard of Yendor has the real Amulet, he gets an infinite mana pool to cast from, or otherwise his magic becomes more powerful.

Whenever the Wizard of Yendor revives, he has immunity to whatever he was killed with the last time.

If the Wizard has started to harass you, enlightenment shows “You are being harassed.”

Instead of the Wizard of Yendor stealing all quest artifacts, he can only steal ones that don’t belong to your role, since you didn’t actually earn them.

Make the Wizard of Yendor’s corpse always taste terrible.

If the Wizard of Yendor’s monster ID is a multiple of 1000, he should be named the “Warden of Yendor”.