It might be technically possible to include a feature that could alter the tuning of any given song lesson. But the practical implementation of such a feature would probably be insurmountable. There are just too many factors involved.

First, the publishers might not even agree to it. Assuming you get that permission, it might work somewhat decently if you alter the key a small amount (a half-step or two?). But once you get too far from the original key it might re-render the audio so much as to be unusable. Think about slowing down or speeding up a tape recording just a little bit versus a lot. Eventually, the sound it going to be distorted, or wonky enough to be unusable.

And adding this feature & ensuring it works retroactively on an existing database of over 11,000 videos & over 600 songs is beyond impractical.

In short, I just don't see this feature being added anytime soon.