i'm listing racial bonuses I think should effect leather armour when skin has those props etc.

Natural armour: This should have a big effect in a +AC way, so harder the skin of the race the higher prot the leather will offer.

body size: maybe just have chance of getting more then 1 skin from a larger race corpse.

immunity/resistances: if they have a weakness its a -AC, if its a resistance its a +AC bonus.

Sight: if helm made of something with darkvision, helm can bestow such?

Magic resistance: I say leather has high protection vs magic damage types

Lucky: increase in defensive skills? like parry/dodge/block? very small bonus

I'm not sure how stats/regen rates should effect things, maybe higher hp regen rate offers more physical prot and higher mp rate is for magic prot?

These are a general quick over view of specials, now for each race i do believe there should postives/negatives for prot. like skin a seraph get good armour but weak vs disruption etc.