The Heart of Ahriman should do instakill when it is slung and hits a monster with a head who is not wearing a hard helmet. To encourage this, Barbarians should get Basic skill in sling.

To compensate for inevitable cheap deaths, any monsters that are too important to cheaply instakill via this method need to be given hard helms of some sort.

Barbarians who go into berserk mode start attacking multiple targets in an arc as if wielding Cleaver.

Barbarians do not require a saddle to ride tame monsters.

Barbarians who roll a 1 on their damage die get the die rerolled and the second result used.

Barbarians get extra nutrition from eating corpses, since they have practice in how to eat them properly. Possibly up to 1.5x the nutrition.

Don’t gift a character a spellbook as a prayer boon if the character is illiterate or certain roles (like cavemen or barbarians), and if a spellbook is given limit its level somehow so that a character doesn’t get a spellbook of much too high level.

An artifact often gifted to Barbarians which provides magic resistance and half spell damage but prevents Pw from regenerating. Or assign these properties to the Heart of Ahriman, which makes it good for Barbarians as they rarely cast spells.

Barbarians and cavemen get weapons/ammo instead of spellbooks as a prayer boon, or enhancements to their current weapons.

Barbarian unarmed skill enhances for free when you finish the Quest, get crowned, or reach a certain level.

A Barbarian rage mechanic (possibly an effect on Cleaver replacing its current damage bonus) where a damage bonus is assessed based on how long ago you last killed a monster. Originally a buff to Vorpal Blade where it beheads anything it hits if the last behead was N turns ago, but this is unbalanced.