With the current model of combat, that's quite difficult to pull off without overloading tanks to the point they're a complicated mess of buttons and cooldowns. Any model with more action orentated combat removes a lot of the buttons, but really does ramp up the pressure on people wanting to play the role.
I think the answer may lie outside the MMO space. To propose something radically different - What if MMO's lifted Tanking mechanics from Fighting Games?
What if your tank played like Ryu from Street Fighter, for example? On face value it seems absurd, but digging into it, Ryu's basic skill set shares lots of similarites with a typical tank. He can block almost all attacks, can quickly parry to launch a counter attack and is nimble enough to evade projectile fire. The cost to all of this is that he is unable to attack at the same time, you're forced to choose when to defend and when you attack.
When it comes to offensive options, Ryu has his basic selection of punches and kicks. Generally quick to perform, with minimal risk attached to them if they're blocked by your opponent. With a little practice, you can combo some of your basic attacks together for added effect. It gets interesting when you consider his special moves though. Both his Shoryuken and Tatsu are extremely rewarding to land, but can be punished just as harshly if used poorly. The Hadouken is also very telegraphed and is easy to react to, so using it well is paramount if you don't want to get a flying kick to the head.
The final part to this is Ryu being able to combo his special moves from his normal attacks. This allows you to help mitigate some of the risks associated with using them, since a basic attack getting blocked isn't the end of the world.
Applying this to an MMO would create a paradigm where:
- You could defend or attack. Not both together.
- You could take small risks with "Basic Attacks" for small gains.
- You could take larger risks with your "Special Moves" for a much bigger pay off.
- There was a solid mastery curve, where you could work on reducing the time you need to spend defending.
- Mastery in being able to improve your offence, first by learning to combo your "Basic Attacks" then by leading into your harder hitting "Special Moves" with less risk.
I appologise to anyone who has never played Street Fighter. I tried to keep this simple but it may well come off as a pile of gibberish. NOTE that I'm not advocating having to input your attacks in a style similar to fighting games here. Just that, perhaps, Fighting games have some design choices we can incorporate.