Would it be possible to add a “random tempo variation” option? Something like ±1% to ±10% BPM fluctuation to make the backing track feel more human and less perfectly steady. It would be great for simulating real jam sessions. Thanks!
Thanks for your suggestion, Manny! I appreciate your request… which makes it all the more hard to say no. 
First, I actually experimented with this a few years ago, and I didn’t find it to actually sound more human at all. I then researched online and came across this published paper where researchers had people listen to music (probably MIDI recordings captured from real musicians) where they had manipulated the timing of the notes to be either more “perfect” or more “human” (varying). I can’t find the original paper, unfortunately, but the conclusion was basically that people didn’t perceive perfect timing as robotic… just “better” and more enjoyable!
I’ve also done analysis of professional bluegrass recordings and found that the timing is often remarkably consistent. This certainly isn’t always the case, but I don’t think the variation is random—rather, it’s musical, in response to something in the song or performance.
Finally, as a practice tool, I think it’s good to strive for “perfection” in your timing as much as possible (knowing you’ll never fully get there) and having a steady metronomic beat is part of what makes it useful, so even part from the above considerations I’m kind of philosophically opposed to the idea.
Hope that makes sense and thanks again for your feedback!