Adding new classes and trees would be easy and they wouldn't need to re-balance the older classes unless they were adding an extra spec to the an existing tree. They would need to make sure that everything falls in inline with whats there if thats the case.

I could see them adding classes in either patches or expansions (the latter would require more to make it worth it of course). A patch would introduce 1 base class with 2-3 different Advanced Class paths that branch of it.