So then, might that same thing have happened with the other aspects? Might we eventually find out that different Aspects have different "Hearts of ..." and be able to use them to defeat some super evil we don't know about yet? Another Old God perhaps. Would be cool if happens or something like it.

I was thinking of that, too. Alexstrazsa asks you to retrieve the Heart of Magic from Malygos, and she's held onto it for the time being. It's most likely that the Heart contains the power and properties of the Aspect, and whoever accepts it is changed into the Aspect.

Much as I'm sad that Malygos bought it, I'm glad that Kalecgos is going to be the new Magic Aspect, always had a soft spot for the blue-haired bishie ever since the Sunwell.

The Heart of Magic is a pretty obvious Chekhov's Gun.

It is an object of incredible power that Alexstrasza deemed important enough to take from his dead body after we killed him. It was introduced into the story "early" on and we have yet to officially learn of it's importance.

If the Heart of Magic wasn't in some way going to show up again, in this case making Kalecgos the next Aspect of Magic, why introduce the item at all?

While I do see where you're going with this, there is another reason for its introduction here. We needed a quest item to bring back to Alexstrasza. Similarly, those four zebra hooves you collected in the Barrens are fairly unlikely to make another appearance.

But hey, it could happen.

Actually those 4 zhevra hooves were used to make Zhevratron - a mechanical zebra-zombie-robot of unstoppable power. Look out for him in the 16th WoW expansion!

Heart of Magic... hmmm...

Heart of Time
Heart of Life
Heart of the Dream
Heart of Earth?

*Hmm...*

With your powers combined, I am Captain Azeroth!

...wait a second.