As the title says, a show/hide all sections at once button would be a nice feature. Not sure where you would add it (right next to the page title or just under it towards the very right would be my first two ideas., but for pages where all someone wants to read is the very first section or wants to just see the introduction and the categories and doesn't want to have to scroll down very much, that would be a quite useful feature.
Also, while I'm not an expert on Javascript, I just tested this on MediaWiki 1.19 and it worked flawlessly, and given the rather simple hooks you've used, this extension looks very downward compatible.
Also, even if you don't want to use the Wikimedia Git Repository, it probably wouldn't hurt to put this extension on a free Github account in a downloadable zip file with the install instructions and the information needed for the MediaWiki:Common.js file.
I don't have anymore advice or suggestions at the moment, but I did want to say I believe that you are a genius, as I have long searched in vain for an extension that does what yours does, and to say I'm quite happy with it is a major understatement.