We know that Zedd has been working with her on the album, and he mentions that he has "10 ideas" that are in various stages of completion, but their busy schedules are what's holding up the actual recording and finishing of these tracks:
"The problem with her is obviously that I'm busy and I tour a lot, but she's probably a lot busier than I am, so it's hard to find time to work on something together. So we've been working on it for over a year now. There's still a lot of work left, so we're definitely gonna work on this project for the next month... I did about 10 ideas, and they were all between almost finished songs up to the piano. And then when we were on tour together in Asia for almost two months, we had a lot of time to go back and forth and pick the ones we really like and work on them. ... But they're still all really rough — it could be five, it could be 10, it could be anything we want."
Sometimes that time away from each other can be a blessing in disguise; we've learned that Zedd created a lot of what became his debut album Clarity while in the studio, waiting for Gaga to finalize her tour details. We'll keep you posted for more on Zedd's work with Gaga on ARTPOP.