I don't know if adding a new spec to each class is the right thing to do (or even makes sense), but I think that there are options that could go explored. As a warlock player, I can say that few things intrigued me like the Apotheosis option. I normally prefer to remain dps, but if Blizzard had left it in as a fully-capable tanking option, I'd probably be tanking for my raid now. The way that they changed it was a huge disappointment to me. The only real use it got for me while I still maintained a demonology spec was if an add got loose or a tank died, I could pop DA and hold it for a little while so the offtank could grab it or we could get the MT up.
I think it's a really interesting design and there's a lot of possibility to look at there. I would much rather build a solid tank set using existing skills (warlocks already have high stamina, mastery by design would be adaptable, and transforming to tank is already in the game) than go the BC route and be a caster in a clown suit to tank (I'm looking at you here, Curator.)
On the down side, though, as people have said, balancing things is difficult enough, and making sure that another (or worse, several other) tanks or healers have similar enough sets of tools without making them totally OP (and spirits help them if they try to make them PVP-viable) might not be worth the trouble.