Well gamefreak keeps saying "2018 or later" for the Switch game, so I'm pretty sure they're aiming to release the game next year if possible, but can't commit to the possibility, hence the later.
I'm guessing the anime has also been informed to wrap up the main plot by 2018 by gamefreak, due to the possibility of the Switch game in 2018. However, I'm sure if the game releases in 2019, the anime will probably add in an extra arc to buy time. I just seriously hope it's something good, like maybe Rainbow Rocket, and not something like DA.
That is definetly a possibility, but that would be kind of weird if we think about it, cuz this world's Team Rocket remains active and RR is supposed to represent what would have happened if the Teams weren't defeated, but TR was never truly defeated and disbanded in the anime.
Maybe they will make a arc where Necrozma the villain?
I just hope they won't come up with a filler arc like DA. If they do i guess i will skip most if not all of the episodes.