In EverQuest II, healers are always in demand. That's great for my healer, but bad for my main, a fighter. Stacking multiple healers can often make encounters easier, because you can heal through high damage faster. Stacking multiple DPSers can make you damage an enemy faster. Stacking multiple tanks just means you are wasting a slot.
This means having more than one fighter in a group isn't ideal, so people avoid it. The solution the game employed when it came out was to allow fighters to have an offensive stance so they do more DPS. The problem with this: it impinges on the class role for mages and scouts. If the fighters do more damage, then no one will want the true DPS classes. If the fighters have the same or less amount of DPS output, then no one will them anyway, because the mages and scouts will be at least as good and have more utility.
This needs to change. It should be possible to have multiple tanks share the task of absorbing damage. EverQuest II has a few abilities like Intercede and avoidance buffs, but they aren't powerful enough to viably allow, say, a Paladin and a Berserker to stand side by side and share damage through an entire battle. It's an important skill, but it's not attractive enough to make a group think of picking up an additional tank instead of a backup healer.
I'd like to see Intercede modified (or a new ability created) such that it would allow two or more tanks standing within a short range of one another to take less damage overall - since they are presumably helping each other deflect blows. Instead of always transferring 100% of the damage from one tank to another, have a chance to transfer reduced amounts of the damage, subject to an additional avoidance check and mitigation on the secondary tank. It should be an ongoing buff (or reset about as often as the average heal spell) and the total amount of damage taken by the group should end up being a lot lower than it would be with one tank.
A second tank would therefore have a similar effect as having a recurring ward up (because the additional tanks are helping deflect, parry, and block attacks) as long as the fighters are close to one another.
Also, additional abilities could be introduced that allow multiple tanks to work together.
For example, add an ability that allows a fighter to drop all aggro and send it to another fighter in the group, and at the same time, create a heal over time effect on that fighter that expires if they end up at the top of the hate list for any mob. Two tanks could then time things so that they ping pong the monster between them and the self-HOT would allow one fighter to recover while the other tank took charge.
Or introduce an ability that allows multiple tanks on a single mob might to reduce the monster's recovery time, because the tanks are presumably working together to keep the mob off balance.
If implemented correctly, a group choosing to tackle a particularly challenging encounter should be able to grab a second tank instead of a second healer and do just as well. In TSO zones, many people always insist on taking two healers, which means healers take up 1/3 of the average group. A few simple changes that allow tanks to "stack" would be a great way to increase fighter desirability while still allowing them to fulfill their class role (taking damage) without impinging on the class role (dealing damage) for the scouts and mages.