Earlier quoted context omitted.
The problem with average per song is that you "use up" words in every new song, so all things being equal each marginal song has progressively fewer new words.
I bet you could get something insightful from plotting "unique words" versus "total words" - That might give a good idea of the amount of repetition over time, the length or quantity of output, and the total vocabulary.
http://www.mdaniels.com/vocab/scatter.png
love your other ideas – hopefully can do them later.