I doubt it’ll be possible to do real-time any time soon, with a result that is nearly useful. Real-time stem separation of useful quality is quite professor-heavy already. Next, it would have to do pitch analysis on the individual stems, and then do pitch shifting of certain keys only of individual stems - all in real time. I doubt we’ll see this on a standalone player anytime soon 
The technicalities aside, I do think changing a song from minor to major or vice versa is too big of a change. While it’s definitely a fun experiment to see what a song is like, the soul of it is quite ruined (take the Nirvana one as an example )