Frankly, I think if people are going to harp on so much about "Legendaries being Legendary", or whatever, I think it should involve Heroic Raids.
If you're not actually going to make it actually really rare and very hard to obtain, just make it an inclusionary timesink.
Imagine if a whole-expansion-legendary instead required at least one kill of the heroic version of every final boss of this expansion's raid to get a chance at a small drop toward a weapon on Garrosh.
That'd be a legendary.
But barring that, I don't see the problem with a very powerful expansion timesink having the legendary color or whatever. Who really cares :x