Will never know for sure until probably about 90-120 days out.
But it is an exciting time. My plan is to go to WDW late 2018, get an annual pass and go again late (Oct/Nov) 2019 and hopefully catch both openings. If not, oh well, I will just have to go again in 2020 LOL!
The fact is, that no matter when you plan on going in the next 4 years, you'll miss the opening of something. I went in August 2015 and missed the opening of the Star Wars Launch Bay. I just went in March 2017 and missed the opening of Pandora.
I can't recall a time when so many new (and net new) things that have and are set to open in the next 1-4 years (in so many different parks) without actually opening a whole new park.
Pandora - 2 net new ride attractions
SWGE - 2 net new ride attractions
France Pavillion - 1 net new ride attraction
Tomorrowland - 1 net new ride attraction (possibly 2 if they ever say what they are doing with Stitch)
Main Street USA - 1 new show
(Toy Story Land is really just replacing Back Lot and LMA).
So 6-8 new attractions between Jan. 2017 and probably end of 2021? DHS and AK barely opened with that many attractions.