Improve "design in the browser" capabilities. i.e. pushing CSS/HTML and maybe JS changes made in Dev Tools back to the source files without manual shenanigans in the middle.
I probably waste more time copying CSS / HTML tweak diffs from the browser back into the related source files than anything else with in browser dev tools. Due to the nature of 'losing changes' if I happen to hit refresh or if the page has some automatic polling mechanism I also tend to do this very often as some sort of manual 'save' action.
There have been a couple attempts to do this (http://www.cssupdater.com/ or https://code.google.com/p/backfire/). But, I'd really like to see a standardized protocol for pushing changes to the server such that server side frameworks can implement it appropriately.
For example a server side implementation for Rails needs to understand the asset pipeline to know how the final file was created so it can work its way back to the correct file to save changes to. This also means you probably need source map style support for LESS and SASS also. Also to change HTML template files the server side needs to know how the page was constructed so it can find the correct partial / template to save changes to. Which is probably really hard depending on how much state based conditional rendering there is in your templates, maybe the server side caches the last response and associated state to work around this.