Will their every be any more genders for the human avatars besides male and female?

I have to say, I wouldn't get my hopes up. With tens of thousands of items that would need to be drawn in case of new body curvatures, I don't think it would happen.

Though there are many ways to create andro or nonbinary characters.

Ah, okay. Oooh! How can I create a nonbinary character?

As mentioned above there will most likely not be any added bases due to having to redraw the wardrobe items to fit a new base. However, as suggested you can make a non-binary avatar by selecting a "male" base and dressing them however you wish. This eliminates the breasts and can then be used then to create a more androgynous base style. There are also multiple facial items that don't have lipstick/prominent lips or eyelashes to help create a more neutral face as well. :)

The Anatomical Muscle tees, even on "female" base has a drapey shape that doesn't show much in the way of breasts.