Using what RatingBuster and some other mods out there do, but going a bit further, I envision something along the lines of a paperdoll in which one can switch out various items (seen on the AH, loot tables, links, quest rewards, etc.) in order to determine a desired outcome of stats and effects.
It would list all critical values based on Class, Level, (spec?), and whatever is "equipped" in the window, AP, Crit, +hit, +spell damage, +heal... the "whole nine yards"
Maby even a way to save sets for later comparison.
Envision as I may, I am clueless to how to code such a thing..
I've thought about this a number of time, Blizzy needs to make the dressup frame show stats as well, it seems to me it should be trivial for them to code that. Us however... we don't have direct access to any of the calculated stats except what you're wearing right now.