When I was a physics student, I would have loved a spreadsheet that tracks units, and automatically does conversions as needed. There is another huge thing that this enables: Tracking uncertainties. It would be awesome if you could not just write "10", but "10 cm ± 1 cm". Then the spreadsheet could do error propagation! Even the simplest case of non-correlated errors would be useful. Also you could have functions like "weighted mean" that take the errors into account. We used to do these calculations manually with extra columns and a few macros, but it was always error prone.

Also, why is there no CAD program that allows me to enter errors? When I try to model real objects, say a room, the lengths never add up 100% and I always have gaps once I'm around the room. It would be great to say all these measurements are ± 1 cm, this one is ± 5 cm, and these angles are 90° ± 1°. And then it would do least-squares, and give me a really accurate model, or tell me where I have to measure again.