Sailing
hmm... kinda hard
I'ma try throw some runescape'esque ideas out on the table maybe you could develop
Astronomy?
(no idea how that would work but meh)
Stamina
(perhaps run-energy related, or how many inventory slots you can have or how much weight you can carry)
Stealth
(controls monster aggro, idk)
Navigation (potentially controls what % of the map you can reach, minimap size, idk
Affection (to do with pets?)
Sailing (perhaps you could own your own boat, sale on the ship, sailing level affects speed / accuracy or w/ever)
Gathering (harvesting wild plants / something of the sort)
Anyway those are a handful i thought of off the top of my head, might edit this post with more...
gl! - apaec