I think there aren't any secret ones. Just some come up at certain dates and if your overall situation is right. With Italians for instance, the Noble Mercantilism comes up just three times, shortly before points are awarded for it. As opposite example, if you are really losing the campaign, and have lost even some of your homelands, you won't get some of the extra GA's like Drang nach Osten..
Another is "capture Constantinople" for the Turkish, I suppose there is one "hold Constantinople" for the Byzantines, now I didn't get that when I was playing the Byzantines because the Turkish were wiped out long ago..