Not much is known about the plot for The Suicide Squad besides the fact that it'll be inspired by 1970s war movies and that a very large team of villains will be attacking a paramilitary base on what appears to be an island. James Gunn didn't reveal anything apart from the enormous cast of blackmailed criminals.
However, before DC FanDome, some pretty huge spoilers were released by outlets. These spoilers, coupled with hard evidence present in the sneak peek for The Suicide Squad shown at the online convention, basically confirm who the villain of the movie will be, though.
Initially reported by Splash Report and later corroborated by The Illuminerdi, the day before The Suicide Squad panel at DC FanDome, it was revealed that Starro would be the primary villain of the movie. While still a rumor at the time, once footage from The Suicide Squad was shown to fans, many were eager to comb through it and find hard evidence of this report.
Their search bore fruit and many fans and outlets found multiple extras in the behind-the-scenes footage with what can only be miniature Starros attached to their faces. The clearest evidence of this is 45 seconds into the exclusive sneak peek video shown at DC FanDome for The Suicide Squad:
Slowing down the video and pausing at just the right time will show, behind and to the right of Daniela Melchior, an extra on the ground with a purple prosthetic on their face. The prosthetic looks strikingly similar to a Starro spore with the purple color scheme and what appears to be the correct number of tentacles gripping their face.
WHAT THIS MEANS
In actuality, fans don't know much about the plot of The Suicide Squad besides the fact that the John Ostrander and Kim Yale runs were major inspirations for the tone, and potentially the plot. However, when answering a question about King Shark on his Instagram, Gunn describes the original Suicide Squad as "...a group of adventurers, soldiers and scientists, who explored and battled anomalous events,” which changed drastically over the years, especially in Ostrander's run. Gunn went on to say that "Our movie takes elements from all those timelines..." including the original incarnation of the Squad that fought "anomalous events."
So, it could be possible that Waller sends the Suicide Squad under false pretenses again just like the last movie. It unfortunately means that the team would also be fighting mind-controlled enemies again, but, only on a surface level. It's typical of Starro victims to be saved by simply removing the parasite from the victim's face, changing the dynamic.
Another funny possibility that was pointed out by some is that Starro could possibly be voiced. Fans still don't know who Taika Waititi is playing, though many believe he could be providing the voice for King Shark. However, he could instead be the voice for the famous Justice League villain.