Am all for achievements to reward me for tanking. However keep in mind that the problem of queue times you address is a very server specific issue. I'm leveling up my off-classes mainly by the daily low level roulette reward, and my average queue time on DPS classes is around 7~8 minutes.

There's hardly a wait already as is. Sure, a few more tanks and healers would be nice. But if the incentive to go in as tank or healer is made to be even greater, I wouldn't be surprised to end up seeing instant pops as DPS and queue times as tanks/healers on Odin.

So rather I'd say if an achievement would be set in place, to let it be a dynamic one. Enter as the role which is currently in need to make it count. Which could be a Tank, Healer, but DPS as well.