I think a horde/alliance split on the level bar graphs would be a good modification. Normally you see a tall bar showing how many characters are at, say, level 15. What you don't see in that view is how many of those characters are on the horde side and how many are on alliance. Splitting the bar into two colors could show the faction split in the same amount of space.
The same display could be extended to the class breakdown graph, and even the race breakdown for the sake of clarity.
Mockup 'cause I'm bored! (Not bored enough to color every level, though.)
This would be a very uninteresting modification if every split was very close to 50%, but otherwise I think it would be a handy change.
