Hi,
Anything you can modulate with a modulator automaticly, you can modulate manually.
It's allready possible somehow to set the Sample/ Grain start, the direction of the sample, the speed of the grains, the pitch, and the 6 samples slots also give further possibilities in playing different samples.
I thought that you might wanted a to be able to change the regular Sample Engines sample playback time so the playback time and the played part of a sample is the same no matter which pitch is played. Meaning that a unlooped sample allways play the excact same amount of time no matter which pitch you play it in, if you hold notes. I Thought that might was, what you meant by "Speed" and "Stretch".
I don't know how much CPU such a feature would require. At the time being the 6 sample slots can be used to contain 6 samples sampled in different pitch. I can see a potentiel in a sample timestretch feature.