The rumoured apple Rider was based on the whole "Adam and Eve" thing, and it doesn't seem like that's gonna happen now that Toei is calling white-Mai "Alternate Mai", pretty much suggesting she is just that and not Eve.
I wouldn't be surprised if there is an apple Rider in future just because it's such a common fruit, but then maybe it's not so common in Japan, reportedly apples are very expensive there ...
I always thought there would be an apple rider, not because of adam and eve, but because they seem to be using Norse themes. The Norse did have the apples of Iðunn.
http://en.wikipedia.org/wiki/I%C3%B0unn#Apples_and_fertility
There are mentions of nuts there too.