This may seem odd coming from Dark Age of Camelot, where heavy tanks soaked more damage and dealt more damage than light tanks and hybrids, but that game's design and balance (mostly with respect to the interrupt system and battlefield mobility) made this a bit of a different situation.
Early on, when gear was a little more sparse and itemization was filled with question marks, you could build a reasonably offensive tank, but the kit with which you could outfit yourself didn't really cater to that sort of build.
As time went on, however, gear improved, abilities were changed, and most of the melee dps classes received a bit of a "dial back" from their original power, either through actual adjustments or through adjustments to everyone else.
The two "perfect examples" of strong tanks in an offensive role nowadays are the Ironbreaker and the Chosen, though we can't forget that the Black Orc had its share of time in the sun (and may be making a bit of a comeback).
The Ironbreaker, via the Grudge system, assortment of utility abilities, ability to wield a great weapon (though this isn't a necessity), and ability to debuff for its damage is a highly capable damage dealer. The Chosen, via the aura system, assortment of utility abilities, ability to wield a great weapon, and ability to debuff for its own attacks, is also a highly capable damage dealer.
This normally wouldn't be that big of a deal, except that these two classes are often capable of reaching offensive potential in line with those classes who are designed to do melee dps, while retaining the ability to gear and spec defensively for PvE and PvP.
Part of the problem and reasoning behind this is that tanks really have no incentive to spec for a tanking role, because most players will simply ignore the tank and go after the more vulnerable targets (unlike mobs who have to deal with things like threat/hate). There is no reason for the average tank to play as a tank, so they don't play as a tank and instead spec for dps.
So, what's the fix? There are a few steps that I think would be necessary to give the ability to tanks to, well, tank, and some of them might require some heavy-handed pushing in the right direction. In my opinion, we need to:
A - Limit the number of tanks that can wield Great Weapons
A fundamental decision seems to have been made to give all tanks the option to wield a two-hander in addition to sword and board, presumably to give them the "you don't have to tank" option, like they gave healers a "you don't have to be a dedicated healer" option (ha).
The solution? Reduce the number of tanks that can wield Great Weapons. How much of a reduction? I'd be apt to drop this whole thing down to two tanks that wield Great Weapons -- Swordmasters, for obvious reasons, and Black Guards, for obvious reasons. You could make an argument for Black Orcs, but these would be my picks.
However, by the same token, these two classes should be allowed to wield only Great Weapons. Given that they are tanks, they probably need to be given some extra toys to allow them to "tank" with two-handed weapons (though as we all know, the Swordmaster is already there, and the Black Guard is pretty close).
Once we've limited ourselves to two tanks wielding Great Weapons, who will be our two offensive tanks, we can model them around the best designed offensive tank archetype in the game...
B - Model offensive tanks after the Black Guard, with some of the Swordmaster's defensive touches
I'm sure some of you are scratching your heads at this one, because the general consensus is that
So why is the Black Guard the best class to model ourselves after for an offensive tank?
First, the Black Guard has reasonable damage output thanks to the use of a Great Weapon (and access to Hastened Doom), and has the ability to pick up immunity to detaunts as well. That being said, the damage isn't high enough that the Black Guard will chew apart a decently equipped paper wearer in seconds, but it is high enough that the healer will need to worry about the offensive tank pushing in and their group will need to worry once the offensive tank is on the healer.
Second, the Black Guard has a solid assortment of harassment tools and ways to handicap/cripple the enemy without relying on huge damage (which is the melee dps' job) -- outgoing heal debuff, morale regen inhibition, an assortment of attribute debuffs (including Crimson Death to make them more susceptible to critical hits), a disorient, a pair of interrupts, AP cost increasers, etc.
Third, the Black Guard has reasonable durability when offensively spec'd as a result of other abilities and tactic choices -- if you supplement this with a few picks from the Swordmaster, they get a good option for PvE tanking and some added boosts in PvP.
People complain that the BG is a bad tank because it doesn't put out huge damage numbers, but out of all six tank classes the BG is really the only one I fear (well, maybe Chosen because of all the debuff-for-my-own-damage-type garbage). Fortunately for me, most of the people playing them mash Monstrous Rending into Murderous Wrath.
So where does this put the other four tanks? If we're taking the offensive focus away from the other four tanks, they really need to be given a focus on group utility and buff/debuff action to make them worthwhile to group and worth trying to kill (where their tanking ability can really shine). While two of the tanks have a decent assortment of both features, two of them really excel in this department...
C - Model the defensive tanks after Ironbreakers and Knights of the Blazing Sun
Yeah, yeah, boo hoo, nerf Order, Mystic hates Destruction.
But let's face it. The Oath Friend system is a great system for supporting groupmates with buffs and debuffs. The Grudge mechanic is a good way to represent hulking up when someone lays some beatdown on your paper wearing buddy (or tries to kill you after you're enough of a pest. The Knight's auras offer a far better assortment of buffing/debuffing than the Chosen's (and don't have that "debuff your own damage for huge numbers" problem). Both classes have some great 'defense-minded' tactic selection.
Now, we don't want to shuffle the mechanics around drastically. Obviously, we can't really throw an Oath Friend-style buff over to the Black Orc (who shares stance dancing with the Swordmasters), but there are ways to accomplish the same goals.
Don't give the classes mirror abilities, but design their abilities with those same goals in mind: protect your groupmates, buff your allies, and debuff your enemies.
Which leads to the last point...
D - Add some defensive/support abilities that are worth using in PvP and that make you want to kill the defensive tank
Seriously. It's not rocket science.
Obviously, the one I'd love to implement is "player hatred" (akin to mob hatred), where if a tank builds up enough hate through hate builders/taunt, they can force a change of their target's offensive target. That being said, I don't see it happening.
So!
Things like Challenge (debuff damage, offset taunt, does not affect the tank) are a good idea of where to go. You want abilities that affect friendlies or enemies but are of little benefit to the tank itself (tanks have enough tank-boosting toys, and if they only have a defensive equipment option they'll be tough enough). The auras having buff/mirrored debuff effects are another good example of a solid ability even if they do affect the Knight.
Heal buffs. Defensive attribute buffs/offensive attribute debuffs. Cures. Root/snare breaks. Stagger breaks. Knockbacks. PBAoE/cone snares (why this is on mdps, I'll never know). Something like a high-valued channeled snare. A defensive target run speed buff (Black Orcs punting people friendlies forward into battle or back away from it = a win).
Design abilities that make these classes wanted in groups. It will make sitting in the back and protecting friendlies an option, but it also allows them to push in and support an mdps class offensively (or maybe even an offensive tank).
Overall, these are the sorts of changes that I feel would be necessary to get tanks "right" in Warhammer. Time permitting, the next parts in this series may involve me going through each tank class and "redesigning" them to fit my "vision" of tanks.
We'll see! ;)