From the little I've learned so far, the plot and designs is already made (by Toriyama) and the story will cover the origins of the Saiyans as a race and the first Super Saiyan God aka Yamoshi, pretty hyped imo!

This pretty much confirms that the movie (is this a first?) will be canon, Toriyama himself canonized the whole Yamoshi / S-Cells thing and now a movie comes from all of it. 

From the Heroes panel, DB movie 20 (counting all franchises) will be released in December 2018. I'll add more details as we get them. Here's the website they registered a few months back which caused the rumors about this movie to start flying in the first place. Now it has something on it. (Not much, but something.)

Article on Toei's website.

English Toei article:
The team behind the new film will be making intensive efforts to ensure that the scale and the quality is the best of the series. Akira Toriyama, the original author of Dragon Ball, will lead the original story, screenplay, and character design.
The theme for this remarkable new film will be “Saiyan”, the strongest race in the universe. Since “Battle of the Gods”, Goku has undergone new forms from Super Saiyan God to Super Saiyan Blue to other evolved forms that have gone up against many invincible warriors from multiple universes. This new story will focus on the origin of the Saiyans’ strength and what it means to be Saiyan.
If you missed the Toriyama interview about the origin of the Saiyans, it's here. Chances are, the details in this interview will play some part in the movie.

As was the case with the last movie, original author Akira Toriyama-sensei is in charge of the original work, script, and character designs, raising expectations even higher. It looks like he’s already done more image illustrations than for any movie before, so we can’t wait for the film’s debut in December 2018!