Some kind of animation for the splitters when you collapse/expand them? Even nowadays there are people not used to the web environment, and if the collapsible pane just hides instantly (or almost) they will first think 'Where did my *whatever* went!?' so having some kind of easing they would see where did it go.
Useful for automatically splitting the major dimension of a screen/view, for example. Especially handy for supporting tablet orientation change. Or, add an orientation() or setConfig() method, to support horizontal/vertical toggle via script.
Live resize option on a splitter so the content scales as I move the bar, not just show me the ghost handle Depending on the content this obviously could be laggy...but let me worry about where I use it :D