I know there's the live API being worked on(third party through midi remote scripts and python) but it's nearly completly useless.
Examples of plugins would be effects, instruments, and midi effects(of course)
On top of that mixer level plugins that would reside say next to the sends. And general Master level plugins.
It would be so useful to have say a plugin that is a small display with a few midi buttons that could be activated and deactivated for example providing visual feedback to whatever is happening(like the mute buttons) or to create a little window which would analyze the audio signal and draw a spectral graph to represent it so we could see the levels. So on.
I think an implementation like this would make live absolutly boom past any DAW(even tho for the purposes it was designed for it already does).
Please ableton, I would invest half of my music making time into writing plugins for ableton
