Beholder corpses have a chance of conveying a Charisma increase accompanied, if you are hallucinating, by the message “Beauty is truly in the eye of the beholder.” Alternatively, do not have a message, but add this line as a comment in the source explaining why a beholder would grant Charisma.

To make them more accurate to D&D lore, all beholders should grudge all other beholders.

Beholder and magical eye corpses should give, or have a chance of giving, +1 to Intelligence when consumed.

If an illithid hero attempts to use their psionic abilities on a beholder, it fails and they get confused instead, because the beholder’s mind is so alien that they can’t comprehend it.

A monster passive ability (possibly for shambling horrors or beholders) that unmaps its surrounding. This would be most useful when the monster also has something that incentivizes blinding yourself when near it, such as gaze attacks.

However, this can be negated by player note-taking of the map before the monster wipes it, so care should be taken not to make that worth doing.