Yea the idea is great. It what gives variation in the tune. But indeed, I'd just stick with one change on the 16th bar. What I also do alot, is fading things in and out. Fading a piano in over 24 bars, while a pad is fading out, stuff like that.
Yea finding vocals is a bitch. I'd love to use more vocals myself, but I just can't find vocal's that fit