I came across this idea on the official wow forums and kind of fell in love with it, so I thought I'd bring it over here just to have fun with. Someone took our age old use of being ranged caster tanks and took it a step further. The general idea is that Demonology would have the option to sacrifice their pet and gain a permanent demon form that increases threat done over dmg. We'd use SP as filler, we already had a taunt during wrath, resistance to stuns, and maybe a slight reduction to damage done (much like warrior prot stance). We have a self healing ability in DL, and demon soul and demonic knowledge could become typical tank cooldowns. The talent tree would be similar to the current feral druid tree (in that it's a split tree between tanking and dps) and bliz could add in a function that converts int to some form of mitigation (dodge comes to mind, having a greater knowledge of your opponents arsenal gives you a better chance of avoiding damage).
Before the trolls start I realize this will never actually be implemented, I just wanted a fun idea to toss around on the forums to pass time. Any input to make this a realistic spec would be awesome even if bliz is too lazy to make it happen.