Blizzard has said that they want to integrate some form of threat meter in the UI. Of course, they have the hate list server-side, so they could do whatever they want and always be correct. Do not expect that addon will get access to that hate list, though.
My feelings is that they will introduce an event that will tell you when you get close to taking aggro from your target. It is sufficient in most cases, it doesn't give away special boss abilities or anything, but allow the dps classes to do their job without having to guess. Yes, it's far from a complete threat meter, it still has issues, but I'm pretty confident we won't get more than that.