Ok, clarify what I'm posting about:
I'm not saying the ENTIRE Horde and the ENTIRE Alliance will merge. I'm merely posing that individuals might be able to swap there primary faction. So Timmy 1 can go Horde as a gnome to play with friends and Timmy 2 can go alliance as a goblin. You would still have the general 'Horde vs Alliance' but individuals may choose to align there allegiance. Similar to pandas.