As far as Gaga, Rihanna and Katy go, I can only see Rih having lots of success in the future.
I don't think Miley will have great success after Bangerz, but we'll see, maybe she'll reinvent herself again.
But I do think the 'pop girls wave' overall is kinda done.

It used to be very easy to get big being a young girl and making pop music, but that's changing fast.