I think that some form of on the fly sample replacement feature would be great - like, being able to define a grid - say 8down and 6across (my setup for live use) and then map a hotkey that would replace the contents of that grid with a "Live Group" - basically a on the fly reassignment of the grid to a new pre-defined set of 56 samples. These grouped samples would be a sort of replacement macro. You'd trigger the replacement either from the library with a doubleclick or from a hotkey.
Any samples that were playing when you activated the sample replace feature would be moved into a new scene, one row below the defined grid and occupying the same track. They would would continue playing, and you could then move into a new set of samples to keep the groove moving on the floor.
This would frankly allow me to totally eliminate all other DJ solutions. As it is, my solution is to mix in a song in traktor, fade out and stop Ableton, load a new set, and then mix out of the Traktor track. Not ideal, certainly, as I hate task switching like that during a set.
Secondly, it seems like some of the pre-defined (and unmappable) hotkeys are suited to production work, while I would love to have those keys available for live performance. Perhaps have two modes for Live - a live performance mode and a production mode.