Since I’m more obsessed with surround sound than anything else these days, I’d have to say a low-cost system for doing surround mixes. And if that system happened to be Pro Tools LE, I wouldn’t have minded.
I’m still hoping for a direct brain-to-computer interface. Failing that, some sort of truly universal control surface that would let you work without need to look at a computer monitor would be a close second.
A universal copy protection scheme that’s totally transparent to legitimate users — and can survive the demise of the company making the product. Otherwise, music software’s full potential will never be realized.
A DAW feature that would analyze the virtual mixer signal path for digital clipping, so you could track down problems and maximize headroom. As things are now, analog is more forgiving to work with.