I play a defender, so be gentle with me

However, I want to see Tankers get what they need and want them to be effective and useful as much as anyone; so I offer this as my suggestion to this discussion.
This may be in the realm of pure fluff, but I think it would be neat if as the villians become more and more concerned about the increasing amount of damage the tanker was producing, a few things happened:
1) Specific emotes from the villians that had increasing amounts of "worry" and "concern" voiced as the tanker continued to increase damage over time
2) Perhaps as Tanker damage increased, create additional effects, passive or active, in a tiered manner that were beneficial -
ex/
Level 1: (base)
Tank does X damage as they deal damage in the 1st 10 secs
Level 2:
Tank does X+Y damage as they deal damage in the next 10 secs, and the damage resistance of any current target drops slightly
Level 3:
Tank does X+Y+Z damage as they deal damage in the next 10 secs, and not only does the damage resistance of any current target drop even more, but other targets in the area of the tanker randomly get affected by Fear - not the way it works now, but the way it will work when it is changed..mobs cower but do not run away, since they are so awed by the 1st class can of whoop a$$ the tanker is dealing out with free refills!
You can keep going with this, (even graphic effects as the tanker moves from 1 tier to the next one above) but I only use these examples to illustrate a point. As you initially said, aggro will increase as the mobs get more concerned about the tanker bashing in peoples heads, but perhaps you can combine that aggro with additional benefits for the tanker, and even his/her teammates to really make a difference and have a true, interactive comic book feel for being a tanker.
As Statesman said, the code to do anything still needs to be developed, so maybe something like this can be included if Tankers feel it would be worthwhile.
Good luck, Tankers - I hope you get what you need.