Something I would love to have is a debugger for an interpreted language which visualises the program's state as a graph, say, and you can wind time forward and back. Then, as you make changes to the code the visualisation updates according to what the program's state would have been at this point in the program had it been running the updated code. To me that would feel like magic.
> Something I would love to have is a debugger for an interpreted language which visualises the program's state as a graph, say, and you can wind time forward and back.
This actually exists in the IDE for Inform7, a DSL for writing interactive fiction (text adventure) games. The feature is called "Skein", and it will graph all your playtests, and highlight in red which ones changed after you change your code. You can see it at 6:20 in this video: http://vimeo.com/4221277
