I think one of the main reasons why this class is so overpowered is that they can more or less completely by-pass the rock-paper-scissors style damage-type system the game has.
2H Swords already have extremely high base damages (higher than all other weapons), but Barbarians in particular get access to skills which provide high scaling types of every different physical damage on a low cooldown, with overheating.
They never, ever, have to change their already strong weapon out like other physical melee classes have to and there is no repercussion for playing in a completely stagnant manner.
So here are some suggestions:
1.) Give a damage reduction to skills while using a weapon of a different damage type than the skill ( - x % damage using a pierce skill with a slash weapon)
2.) Make all Barb/Highlander skills Slash based (like how all archer skills on the archer tree are Missile type, hoplites are pierce, etc.) so they are required to go into other skill trees to get the damage skills they need/want.
3.) Reduce the attack speed on 2H weapons (in most games, a 2H weapon by logic is not as fast as a 1H weapon) to make them choose between high burst damage and DPS.
4.) Spread out the skills from Highlander into Barbarians and remove the amount of skills that Overheat. Highlander C3 has 8 damaging skills and 4 of them overheat. Barbarian C2 has 5 damaging skills and 4 of them Overheat.
5.) Remove self-buffs from Highlander/Barbs or distribute them to another new Swordsman branch class.
I don’t particularly like this suggestion, but it is an option. Clerics have strong buffs which help solo if they decide to go physical damage as monks, but otherwise are aimed towards team-play. If you can just Concentrate/Gung Ho/Frenzy/Warcry yourself, why do you need someone else to Blessing/Sacrament/Divine Stigma/Deprotection Zone for you?
6.) Allow 1H-spears to be a default available weapon for swordsmen rather than a peltasta/hoplite attribute to give them access to pierce damage damage early and promote weapon swapping.
