There's going to be a track (possibly two) on Giorgio Moroder's album which is supposed to come out in the spring... but it's rumored the Britney track is going to be the lead single so it might come out next month!
and then there's the Iggy collab which Iggy confirmed on twitter yesterday. It's not for Iggy's next album (since it comes out like next week) so it must be for Britney's, which means Britney probably has a good chunk of her next album recorded!