Using a cursed scroll of light to create darkness has some immediate effect that improves stealth in some way.

A more in depth stealth mechanic. Involving hiding in containers, possibly on the ceiling if you can levitate, making monsters behave as if they can’t see you even in line of sight, creating illusions or using displacement to make diversions.

Artifact luckstone Somebody Else’s Problem that confers invisibility and stealth while carried.

Pets are not stealthy by default, and wake up sleeping monsters just like the player does.

Silence auras that prevent both you and monsters spellcasting. Maybe also a directed silence spell, which silences an individual monster. While silenced, monsters can’t be chatted to, and nothing they do will make noise (possibly only if in a silence aura).

While under the effect of silence, the player’s stealth is elevated.

Backstabbing can additionally occur if you are stealthy and the target monster does not correctly know where you are.

Augment the stealth system by adding a monster AI attribute representing “hasn’t sensed you yet, or has lost sense of you”. Also allow players to take a deliberate action to conceal themselves. This would allow for players to hide in (or flee into) a corner or closet and allow a monster to pass them by, then get the jump on them afterward (perhaps in the form of a bonus for attacking an unsuspecting monster). Some monsters should ideally be able to conceal themselves as well, perhaps even by casting illusions that make a door appear as a wall or something to the hero, until discovered.

Invisibility works completely when the invisible monster and the detecting monster are far away. When within 5 (or 4?) squares of each other, its location is known to within 1 space (fuzzing the real location by 1 space in all directions). When the invisible monster takes a direct offensive action, or with some probability when it is adjacent, its position becomes known. It still appears as I, but the I will move wherever the monster goes until it becomes 5 or more spaces away, at which point invisibility once again works fully. This could work differently if the invisible monster is stealthy.

Archeologists should start with searching rather than having to get to XL 10 for it. To compensate, they now get stealth at XL 10 instead of 1.