..........but clearly his music and that of many other solo artists is monotonous as you say,we need more different solo vocals.
MY POINT, OTHERS CALL ME HATER.
Have you ever considered that, the same way you find his music monotonous and that you want more features, others might just like his music the way it is? So why should he disregard those people to appease the likes of you? ponder on that.