I want to see something where we have a big piece of code, and a big standards document it purports to implement, and the system can answer questions like "is this part of the spec implemented? Where it is implemented? What does this piece of code mean (w.r.t. the spec)? If I implemented this part of the spec, where would the changes go?"
How about people just write the spec and AI give us the code on that, that would be mind blowing.
Less immediately plausible, though.
A system for converting a natural language specification document into a formal specification would be interesting.