At the moment 5 gear pieces (4 w\ 2 handed) allow cap of 25 glowing (75% pierce), 25 stained etc etc. & max +50 to skills gemmed, +25 attack & +25 defense gemmed.
Helmets would be cool but extra 5 crit (would bump max crit to 54% lol), 5 glowing (90% pierce on WW3 excessive), 5 attack etc etc would cause imbalance imo.
If helmets were added this set up would need restructured, or the helmets would need to be unique and their properties unrelated to runes & gems.