My idea is to give us two additional choices: Body Height and Body Width. There would be 3 choices for each.
For the Height choices: the current heights would be the tallest the characters could be; with the shortest being a head and a half shorter, enough of a difference without being too small; and the middle height halfway between. For the short races (Dwarves, Gnomes and Goblins), the differences would be far less, maybe a half a head difference (half-head, quarter-head, normal height) but still noticeable. Using the Human as the baseline example:

For the Width choices, they'd be a little trickier. Generally it would operate the same as Height, current size is the largest, and two sizes smaller. Something like a 20% decrease in Width between widest and middle, and 20% between middle and slimmest. For the curvaceous females, the hips and breasts would also be reduced. The slimmest women would be Pettaneko, and the largest would be Boobs of Steel with Hartman Hips. The males would go from a bit weedy to average build to the normal steroid-infused madness. For the short races, the changes would have to be less drastic, and in the case of Blood Elf bfemales the normal size could be the middle, with a somewhat thicker body for Large and an even slimmer build for Small (the differences would probably be the least drastic).

And as a final comparison, all three options used together:

(Of course, the heads wouldn't vary so much ^^; )
And with those additions, the world would become a bit more varied place. Teenagers fresh off the farm would mingle with aged veterans who've seen it all, women can look down on men (some of them, anyway XD ), and more variety would make the game feel more varied and interesting.
What'cha think, and what ideas do you have?