I don't see it happening. Really, developers would have to develop new UI and AI to accomodate it, and increasing the number of characters an individual would control would simply lead to developers to make more 25 man instances instead of 5 mans, and 125 man instances instead of 25 mans, etc. There would have to be difficulty adjustments, class adjustments, probably even graphical adjustments.

However, I do think there is currently an untapped market out there for a good party based game. Haven't seen one of those in years.