@Carubourne:
Well, my point was that it would be nice to not be an existing Execute. Demonology, for example, spams Soulfire, whereas Fire Mages just get a passive increase. Then you have the Hammer of Wrath/Kill Shot vs the real execute (which also didn't scale well enough to be smash this expansion, sadface). A unique and new way to bring in that damage bonus, keep it weaved in, would be kind of cool.
I really really don't think it needs to be a DoT though. Shadow has enough to manage, and letting it continue ticking on below 35% is just a bad idea. :-\