Wow... that's a big question...
I'm not sure this exactly meets what you're after, but I really wish that Excel did a better job of recommending and creating effective charts by default. Currently the charts made need to be customized to fit with best practices (removing a lot of unnecessary elements), and there aren't really good recommendations to push users toward using the RIGHT chart to display your data. (Hence all those pie/donut charts out there.)
I'd like to see more customizability in the icon sets, allowing me to upload my own that are applicable to my business (and allow things like green down arrows and red up arrows, where only the opposite is possible now.)
The ability to load a map of my property into Excel and plot my data on it would be cool too. For example I'd like to graphically represent the sales prices of real estate lots on a topographical map. Although I think I know the answer, it would be interesting to see how altitude and view lines affect sales price. Likewise, what areas on our golf course attract most business from our beverage carts. We had mapping in Excel 97 as I recall, but I unfortunately didn't have data that I could use with the feature. Now I do and the feature isn't there.
Hopefully that gets the discussion started.