It would be nice if it could solve systems of equations. It would also be nice if it could do basic Calculus:

Object launched at a velocity of 13 m/sec on a heading of 34 degrees with gravity of 9.8 m/sec^2 = how far? How high? How long to reach peak or end point?

Then there's derivatives and so on. All that good stuff :) The calculator should also offer all major unit conversions - feet to centimeters, for instance - and sums of integer sequences, integer square sequences, etc.

I have half a mind to go program one myself for fun :)