A device only Steven Hawking would be able to figure out could be difficult to market.
I think you could certainly use this to make unique and creative timing variances however you could also have a serious mess if you weren't careful! I for one would like to have something like this but I think that it would be a really complicated option for newer users and people who are less musically daring! Could you not program to Matrix or RPG-8 to do many of the things this idea may accomplish? As far as speeding or slowing the audio down contained within the combinator (assuming it's not playing back MIDI data), could you not create an insert FX or RE that when feed audio, could slow or speed up the captured sample with the twist of a dial? To speed it up, the whole file would have to be stored in RAM I would think so that it could be processed faster and independent of the master clock and all your other sounds..
Also, if you could do this and in effect have one track playing back at its own tempo, could you then specify a loop area that only that Combinator would follow independent of the track playback and loop points etc? If it was playing back faster, that Combinator would finish playing before the track was finished. That would be tricky to program, at least for me!