An unconventional tank or heavily-armored yet potentially squishy dps, who excels in draining enemies, gaining in ability what they lose, and in gaining strength from perilous situations. In terms of tanking, they are like the old form of Warrior-tanking (healing-only Inner Beast) taken to an extreme. (But imagine also if Maim gave the Marauder a speed or damage buff precisely because it did in fact maim the opponent.) Their attacks are capable of consuming their own health (a percentage of maximum health) for bonus damage based on strength, and their life drains are capable of both (though not simultaneously) increasing maximum health and current health. They have excellent AoE, of which Overpower is just a small part, and high reactive burst especially when tanking. Because of their nature of gaining from enemy debilitation, they act decently as support, but this is the only role that they can maximize without burdening their health or TP. Played correctly, a Berserker is a terrifying asset to a party, but playing a Berserker correctly also requires the coordination of nearly the whole party. In either case it is generally in the middle of everything, whether preparing to swap in as a snap-tank, tanking fully, or wrecking general havoc.
Berserker has the option of using a single two-handed axe or two one-handed axes (aesthetically speaking, polearms such as the short halberd, pudao, bardarche, and volge, and certain swords, such as falx, flachions and cleavers are also usable; the less axe-like, however, the more likely it is that the weapon will be specifically linked to Berserker, rather than to Marauder). Their attacks and priorities differ slightly between the two styles, and Marauder animations have been reworked for dual-wielding when so equipped. Generally dual-wielding tends to work better for AoE, general support, and opportunistic burst, while two-handed weapon use tends to work better for beating down large enemies, penetrating heavier defenses, tanking a single enemy, and AoEing only 2 or at most 3 enemies at a time, though with enough care to one's rotations either style can serve any role.
Berserker is about the only job that can run itself out of TP within a couple minutes without AoEing. It has access to Invigorate, but even then it must measure on how many of its opportunities it should restrain itself. It has proc capabilities similar to Bloodletter's, but it costs TP (likely despite being off-GCD, though I'm not sure if I'll actually go through with that), and triggers from any critical strike, rather than just those of your DoTs.
Berserker's AoE rotations center around cleaves, attacking the enemies critically struck by said cleaves, chaining debuffed targets into their order of attacks between or with optimal attack angles in order to retain debuff-derived buffs, blood-splatter, and procs. Its single-target rotations are similar to that of Marauder or Warrior, except that it can occasionally use an ability out of combo, and has access to Reave (the crit-proc ability), and oc
--Hmm most of these job abilities are turning out to be passive... gaining attack/movement speed from hitting an enemy you affected with slow/heavy, damage from an enemy you affected with Storm's Path, etc., passive bleeds on crit that last for a duration based on damage and can be absorbed in order by Fracture in order to lengthen the debuff's auxiliary effects (although potentially reducing its dps or forcing you to rebuild the buff), and regaining TP from critically striking a bleeding target (with TP gain based on damage), and some means to building or guaranteeing critical strike chance (perhaps just each strike and bleed tick against/on a bleeding target that doesn't reset the bleed duration [doesn't crit] greatly increases critical strike chance against that target). And of course that damage increase with critical health, and that whole lifestealing thing...
Actual abilities... let's see...
Reave (20-second CD) is an off-GCD ability (a bit longish for an off-GCD; takes over half a GCD to be able to use another ability) that costs 80 TP, resets CD with any critical strike on a bleeding enemy, deals 180 potency with an extra 30% chance to critically strike and allows you to use a combo-move regardless of current combo at half its bonus and full status effect at additional TP cost. Bonus combo-move availability is not consumed if you use something you could have used anyways / already combo-ed to, and lasts for a maximum of 5 seconds. Additional TP cost starts at 30 and increases to 40 as time since the start of the buff increases. Attacking the target of Reave with this kind of move refunds 15 TP.
Rampage (toggle) causes critical strikes to heal the Berserker and increase maximum health and apply a bleed debuff with duration based on damage (where 4 seconds would be equal to the base damage (determination not factored in) of Heavy Swing), and causes each critical strike to increase the Berserker's critical strike chance by 6/12/18/24/30/35/40%, attack speed by 4/8/12/16/19/22/25%, at cost of 2/4/6/8/10/12/14% damage. Effect is reduced by 10% (of whatever rank you've reached--i.e. 3% of 30%) for every second since the last critical strike. (A full buff would be reduced to 20% bonus crit and 13% bonus attack speed after 5 seconds without a critical strike and gone after 10.) Stacks drop off every 8 seconds one-by-one and Rampage resets completely if there is no critical strike for 12 seconds. Rampage can be turned off in order to give a buff called "Satiety" which gives twice your stacks' critical strike chance and an equal percentage in critical strike damage bonus on your next attack within 5 seconds. This attack will also heal on critical strike.
Madness (2-minute CD) will cause combos to form randomly and your abilities to attack random nearby targets, increases movement speed by 10%, slightly increases melee range, and causes the Berserker to rush at random nearby targets (any attempt can be stopped, though it will cancel a free auto-attack with it) while providing free auto-attacks. Similar to Vengeance, it comes in two parts: Madness (random combo formation, attacking random nearby enemies, and movement speed) and Havoc (rushes random enemies, free auto attacks, increased range). Rate of free auto-attacks is one fifth of weapon speed. Gives time equal to twice your weapon speed (10 free hits), rounded to the nearest multiple of 2.5. Havoc can be restarted even when on Madness is on cooldown by consuming Rampage stacks, 1 stack per 2 hits. It will reactivate using a maximum of 3 stacks at a time. To use it in this manner, just hit Madness while it is on cooldown; this use will not reset the cooldown, though the cooldown will not decrease while the buff is up.
Sunder (toggle) causes Heavy Strike and Maim to have a small cleave radius, and Storm's Path, Skull Sunder to have a slight line of attack, and Storm's Eye to have a short cleave followed by a line of attack, and Butcher's Block to have a conical attack. Sunder attacks strike (including main target) one target per 75 potency in the original attack, deal 50 less potency per enemy struck (including main target), and costs an additional half of TP cost.
Bloodlust (toggle) causes you to consume 3% percent of your maximum health for additional damage each attack (like the bonus damage of Sword Oath, it is technically magic damage). While active each attack reduces the cooldown on Bloodthirst by 2.5 seconds + .01 seconds per potency of the attack, self-heals are given a chance to crit and are affected by determination, and Bloodlust's critical strikes on bleeding targets regenerate additional health equal to and stacking with Bloodthirst.
Well that's already been 15 minutes... Stopping here.
No, I haven't really been thinking of balance in terms of real numbers yet, (all this is just a 15-minute pipe-dream, after all) though I do believe it can be made to be balanced. It also shouldn't be too Overpowered in PvP; as long as you keep it from managing to debuff anyone, it can't significantly buff itself, and if you keep it away from being able to attack the same target, he can't quickly debuff it. Basically, if you don't let him walk all over you, he can't do a damn thing.