In the new Chromie scenario, you have the option to make your companion Chromie a tank or a healer. The tank mechanic is particularly neat, and should be stolen by every game which uses a companion.
As a tank, Chromie loses 2% of her health every second, until she hits 50% health. But when her health is above 50%, she gains a significant scaling damage bonus. At 100% health, she dishes out crazy damage.
As a healer, this mechanic is quite fun. Normally in games with companions, you spend most of your time acting as a poor man's dps, occasionally throwing a heal on your companion. And it generally has to be this way. If normal mobs hit the companion hard enough to require significant healing, dps characters would not be able to cope.
But with Chromie, a healer can spend most of her time healing, and this is optimal gameplay for her, as the loss of the player's dps is more than made up for by the boosted companion dps.
If you're dps, you get a tank, but one which starts with half health, pushing you to kill things faster. Though I imagine dps players would still prefer to use a healing companion or another dps companion.
I think this mechanic would be an excellent match for a game like The Old Republic in particular, where companions are an essential part of solo gameplay.