Being completely random here.
You get 1 ability from this list at level 1 and gain more (though not all) as you level. You can choose, obviosuly, and some might require you be of certain level.
Beastmaster: you gain your pet with bonuses when attacking the same target and Takedown attack.
Elemental master: you gain 3 modal abilities that change all the damage you do into fire, ice or lightning. You also take reduced damage vs those damage types (either automatically or on per modal basis).
Archery: you gain +6 to accuracy with bows and crossbows and one attack that hits everything in its path (until it hits a wall). Unlike various spells attack area is pretty thin but long.
Slayer: you attacks ignore half(?) damage reduction from 1 choosen creature type. Add another type at later levels.
Sniper: +30% crit chance vs distracted targets (flanked, unaware of your presense or currently under attack of your pet).
Venomous blast: you gain an attack that explodes when it hits your target. Everyone nearby take some piercing damage and become Paralyzed for a short time, then Dazed for a bit more time.
Barrage of arrows: you gain an attack that hits everyone in a medium-sized area with +10 accuracy.
Spellbreaker: An attack with +5 accuracy that deals normal damage and removes 1 random buff, ending its effect completely, from all nearby foes. Removes more buffs at later levels.
Geomancy: medium-sized area effect that slows down foes decreasing their attack speed, movement and melee accuracy a bit. It also breaks all traps in the area for which you have sufficient Mechanics skill. Summon golem at later levels (basically, a tank with immunity to mind control and +10 defense vs magic).
On guard: your movement increases by 1, reflex increases by 5, and each combat encounter the first attack against you misses. You automatically retaliate against such an attack if possible.