Gaga will keep promoting Joanne next year + SB + tour. 2018 ASIB movie release + promo + soundtrack. 2019 maybe new pop album or a new one with Tony (if he's still alive)
I feel like Katy's releasing really soon (near the beginning of 2017).
Then Rihanna (It's hard to tell when her next album will be, probably in the next 2-3 years).
Then Beyonce (probably 3 years from now near the beginning of the year).
Then Lady Gaga (maybe 3 years from now near the end of the year).
