To keep things as similiar as possible to the old berserking damage, I think all we need to do is have berserk damage have 5 base damage added per point of strength, so at 30 strength berserk would put you up to 35 strength with the addition 5 strength bonus which would add 175 to your base attack, which is pretty close to the old system, but it would be scaling rather than needing a cap so every point of strength would be effective rather than only noticing a difference in damage at certain caps.
THAT said, if your toning down damage for some of the other classes, you may want to tone down berserker damage as well. That said, perhaps one way to do that and give more incentive to leveling up a barbarian is to have berserk scale with tier. It used to just increase in duration and that was the only incentive.
To lower damage at level 10, why don’t we make berserk add 3 base damage per strength, that way at 30 strength, boosting to 35 while zerked, you would add 105 damage to your base. At tier 15 the damage bonus would up to 4, at tier 20 the damage bonus would up to 5.
As well, the duration of berserk used to increase with tier, I think if you add 30 seconds of uptime per tier, you scale just about the way things scaled before.
Barbarians also had Crushing Blow if I remember correctly, which was basically just another chance at a critical strike. If critical strike damage is being lowered to 4x damage instead of 10x damage, perhaps crushing blow should do 8x damage and have a much lower chance of happening than crit, perhaps even only being able to be hit while being berserked. It would basically be the barbarian version of a lethal between that and Thunk off of bash.