Actually you lost stacks in ARR if you drop your defiance and turn into "dps stance"
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Fell cleave is 50 gauge for 500 potency, but it also costs a gcd, our average gcd potency is around 200 excluding fell cleave so it's only around 300 potency gain (350 max if you're trading a heavy swing for fell cleave) for 50 gauge. If onslaught gets changed to 10 gauge it'll be better than fell cleave already.
Nothing is being buffed until the end is savage unfortunately.
Onslaught at 10 gauge oGCD makes it much better than Fell Cleave which uses a GCD.
At best, Fell Cleave is a 350 potency increase over our lowest weapon skill (i.e. Heavy Swing) for 50 gauge.
If onslaught was changed to 10 gauge, it would provide a net 500 increase for 50 gauge, hence it would be better fell cleave.
Actually, using upheaval and Onslaught (400+ potency) for 40 rage is already a dps increase over Fell Cleave for less gauge.
The reason Fell Cleave is stll better is 1) being a burst, it benefit more from party buffs (e.g. Trick Attack) 2) it just feels good.
WAR was the first class I played seriously and losing bloodbath, while DPS have it, is really unfair.
Potential changes I would make.
Defiance/Deliverance
No longer costs any Beast Gauge to swap between them
Onslaught
Potency increase to 125; cooldown increase to 30 seconds; generates 10 Beast Gauge
Bloodbath
Generates Beast Gauge per hit, replacing the healing factor it once had; 120 second cooldown; replaces Shake it Off
Fell Cleave
Increase potency to 650. (maybe 700)
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I know that last one looks insane, however I agree with Xeno's philosophy. 500 potency just doesn't cut it anymore due to the buffs Paladin received. The easiest way to remedy Warrior's damage is by buffing the strongest move in its arsenal, especially since the devs decided to base the whole job around Fell Cleaving. As for Onslaught, if the BG increase is too much, just make it Plunge and call it a day.
Look, if you argue onslaught is worse than plunge I'd agree, but saying 10 gauge for 100 potency (if it's changed) is worse than fell cleave is just dumb. The only thing fell cleave is better than that is the lack of cd so you can spam it during berserk. Outside of berserk a 10 gauge 100 potency move is better unless it costs you a fell cleave during a certain phase (you have 40 gauge when fight ends and your last gcd wasn't a fell cleave for example).