Google Wave, for all its shortcomings, had some very useful features. One that intrigued me, and was probably not even noticed by those who tried it, was the playback feature. What if this type of feature were available in other programs. What if it were a part of your VCS or your editor?
Vim version 7.3 introduces "persistent undo", a feature meant to make it possible to undo beyond the changes you made in one session. Essentially, you could undo your way back to the creation of the file. This feature could very easily be converted to work like playback. Having the undo file is almost like having a very low granularity versioning system per file.
The technology exists. What can we do with it? Why would you need a playback feature for your writing or code? Seeing how replays are used in other fields can provide some suggestions. In sports, for example, they are used to look for technical faults and weaknesses. Similarly we can look for patterns that cause inefficiencies in our writing or coding styles.
We could also improve our understanding of our own creative process. Looking at how we go about tackling certain problems will give us insights about how we think. We could look at other people's workings and incorporate them in our own actions. We live in the future; the possibilities are endless.