I think this generation of pop gerls are at a stand still. Lady Gaga, Katy Perry, Rihanna, Ariana and now seemingly Miley et al are just not cutting it for me. None of them have done anything ground breaking with their last efforts.
A lot of them are just in that uncomfortable stage of having been around a long time now and trying to re-define themselves. Gaga, Rihanna and Katy are all in that boat - wanting to evolve their sound after having become staples on the pop scene. Someone like Arianna had less to fall as she was never as big but a) gave us the MEGA BOP Into You recently so she's partly off the hook and b) turning into a features chick for nicely sung choruses was a deft career move.
Leona Lewis should have done this went things started to go tits up and Rihanna's an expert at hijacking every hit song for a chart credit.