Logic (at least up to first-order logic), set theory (some is covered, but not its connection to logic), game theory (essential to so many things, not covered at school at all), and graph theory (the basis for pretty much anything involving computers) would be at the top of my list. Anything where proofs dominate, rather than rote application of rules (we have machines for that now!) would be nice to see. Probability is already well covered in the UK, not sure about the USA. Statistics would be helpful to pretty much anyone.
Don't drop the calculus, but you can teach people to understand calculus in a couple of months. Having them spend a couple of years going from being a thousand times slower than a computer at solving differential equations to being 500 times slower isn't worth it. It's not like simple arithmetic, where getting a calculator out and typing the problem in can be a bottleneck.