I think there is room in the game for leather helmets. The first thing I want to say before you read all this is these would be a kind of item you have to make rather than find or buy, like tasers or blast/kevlar balaclavas and tabis. So keep that in mind while reading.
If you're not wearing metal helmets, you're wearing balaclavas, psi-headbands, goggles, or some specialty thing like gas masks. There's no particular item for lighter-armored characters who aren't psi and aren't ranged, except for some specialty resistances from the various balaclavas.
Edit: I guess if your armor penalty from armor is low enough, and you have nimble and armor-sloping, you can wear a tichrome or super steel helmet, but that's taking a feat for just a few extra DT/DR, and then there's kevlar balaclavas with their 2 DT (6 vs bullets).
A leather helmet would have two slots; leather and padding. I see them as not protecting the face/eyes, so no visor (which would infringe on metal helmets too much, I think), so while they give a crit chance reduction, it's not as much as with metal helmets. I think a detection penalty is okay, but not a perception one. Uninfused leather wouldn't do much, and they'd give protection no higher than un-reinforced boots, and probably even less.
>cave hopper: low mech resist, low penalty
>rathound: low mech resist, low cold resist, tiny persuasion penalty
>pig: moderate mech resist, higher penalty
>siphoner: low mech resist, low acid/heat resist
>mutated dog: low mech resist, moderate acid resist, moderate persuasion penalty
>ancient ratound: moderate mech resist, small amount of DT, highest penalty, moderate cold resist, moderate persuasion penalty
>soft padding: default
>solid padding: raises mechanical DR and crit chance reduction, higher penalty
>insulated/galvanic: obvious
But infused leather would give the benefits of regular leather of it's type (in addition to the higher quality):
>cave hopper: +1 agility
>rathound: higher mech/cold resist, small mech/cold DT, higher than what it's higher quality would indicate compared to the uninfused version
>pig: +1 con, small mech DT
>siphoner: moderate acid/heat resist/small DT, bonus to dodge/evasion like uninfused boots/armor
>mutated dog: higher acid resist/moderate DT, small mech DT
>ancient ratound: higher mech/cold resist/DT, higher than what it's higher quality would indicate compared to the uninfused version
And then we have some new leathers to consider.
