I’m not sure that they’ll announce the next one. I think BCH was a bit of an anomaly i this regard. At the time of the fork, Coinbase announced that they wouldn’t support BCH, and then in response to the outcry, changed course and said that they would.

I’m not so sure they’ll do the same with upcoming additions. I do definitely see the merit of trying to guess their next move though, as there seems to be a lot of money in Coinbase just chasing whatever comes up in hopes that it will also go to 20,000/coin