I don't have a problem with her being the final villain, its not like Madara had a world of development either, he was just hype for most of the series and even after his entrance he was mostly just about being OP, it wasn't until the Edo Hokage got revived that he got any development.
Kaguya will get her development in due time, the real question is how will Kishi handle her.
Also Madara was just a super powered carbon copy of Obito, he basically ran along the same lines, hopefully Kaguya will be a little different in fighting style and how she handles things.