i'm in no way technologically savy, but i grew up with a little game called MTV music maker. i spent YEARS just screwing with it, but as we moved into windows xp, 7,8,vista,10, making it compatable again became too much trouble untill its just junk to me atm. ive heard of music maker2, and possibly a 3rd, but not sure. perhaps you could break down MTV music maker code to find the riff transition smoothing you require?
also, i know you said you would add more selection, considering technically its only one song atm, jsut broken up so u can have a little control. i would be extatic if you were trying to create a 2016 of your own "MTV music maker" if you put in the time to add the hundreds of riffs and etc.
may i also suggest at the time that you solve the riff transition smoothing your export button instead converts the song to a mp3 or what have you as mtv did so we could listen to our concoctions in our audio players.