My kids haven't progresses to altering lyrics, but my son used to mosh, before he figured out that that's why his head hurt a lot. Of course, he did it in the kitchen, when he was the same height as the counters.
His new thing is to tell us his stomach is either doing the cha-cha, or is mad at him. I don't know where he comes up with even half the shit that comes out of his mouth, but I truly admire his creativity. Hopefully he'll keep it as he grows up.