I would love that. This idea came up quite often in these forums already, which is not surprising, as it is just a better version of inferno mode.
You could have a 4th mob modifier, (you could even implement some new modifiers for that difficulty) and you could make every single mob part of a unique group or champion group.
You only have to make sure, that the mobs will not give you more exp than Baal runs and the drops have to be nerfed there aswell a bit I think.