I would like to see the ability to have symbols (circles, squares, etc.) with proportional area that also have graduated colors. Essentially, this is bivariate proportional symbols, with sized based on one variable and color based on another variable. This same capability would be useful in bivariate graduated symbol symbolization as well (size on one, color on another).
Note: This can be achieved using some field calculations and representations. I have written up one such process in Ask the Cartographer. However, it's much more complicated than it needs to be, and it should be easier because it is a useful technique in many situations where you want both a quatitative comparison and a qualitative comparison.
One key aspect of this bivariate symbolization needs to be the corresponding symbolization of the legend entry. It needs to be something reasonably compact, probably with symbols down the middle, size value labels on one side, and color value labels on the other side.
The current multi-variate symbolization results in a table of contents that basically repeats the full set of graduated symbols for each class of the second variable. The is both huge and ugly, a combination that makes it next to useless.