Well lets see-
There are unuseable photomode locations showcased in the Start Menu. That is probably the next unlockable secret. Id be fine with that. IMHO... the best hidden secret would be Stunt Tracks. Remember them being talked about in a rumor news GTP post? Everything in that post came to fruitition but that. Imagine if it was hidden away in a secret menu... I'd give my right kindey to do a barrel roll through a flaming hoop in my Maserati...