I was going to call it Guard Dog, but that's taken. Guard: The pet will automatically attack and taunt mobs which attack the Guarded target.
This would have a limited duration or chances to activate in a given time, something to make it less mindless and less prone to abuse. BM hunters or tenacity pets would have more charges or shorter cooldown, hopefully to the point that a BM hunter with a tencity pet can be a decent heroic tank.
Iapetes and I have been batting this idea around of expanding the available tanks by allowing DPS classes/specs to do some moderate tanking. I didn't like it much at first, but it actually might address a serious problem: the shortage in heroics and the excess in raids. This would add tanks who are good enough for heroics, without changing the supply in raids.
Still, evidence remains that tanking isn't a matter of availability. We have DKs and there's still a shortage. We have dual-specs and there's still a shortage. Druids can tank in their kitty gear. DKs can tank, with a mitigation/avoidance loss (but still more than enough for heroics) with their DPS specs. Tanks have instant queues, which are themselves indicators of a shortage. Something about tanking just isn't attractive to the majority of players.
On the other hand, there is almost certainly a non-trivial minority of players who would tank if they had just one more spec or didn't need to devote a spec. I had the idea of a dedicated PvP spec; a third spec not intended for PvE and purchased with honor or arena points, but I have no idea how to prevent this just being a third PvE spec. Iapetes suggested that the crit-immune talents coming in Cataclysm could be low enough that DPS specs could take them. This would likely be overpowered in PvP or require coding to disable them in PvP. And if DPS are seeking to maximize their damage, they would need spare points to pick it up unless it was tied to a damage/tanking talent already, such as divine strength.
I don't think adding more tanking options will fix the shortage, but it would at least help a bit. And who knows, maybe I'm wrong and there's actually a huge number of players who just need one more spec or less grindy gear options or a pure class player who hates leveling too much, but a respec would work.