Transformers: The Last Knight will (supposedly) be director Michael Bay's last Transformers movie, as well as the film that kicks off the Transformers Cinematic Universe; wherein the Transformers franchise becomes an annualized event, with either a new sequel or spinoff releasing every year. Not much in the way of official plot-related information has been released for The Last Knight at this stage, but fans have been able to extrapolate some details about the movie thanks to Bay continuously confirming specific Autobot, Decepticon, and/or human characters as being part of the film's narrative.
The Last Knight, for example, looks to build on the ending to 2014's installment, Transformers: Age of Extinction, which wrapped up with Optimus Prime (who will, as always, be voiced by Peter Cullen in The Last Knight) setting off to find the Autobot and Decepticon's mysterious Creators. Optimus was previously seen on a billboard for The Last Knight on display in Time Square, showing the Autobots' fearless leader battling a mysterious opponent who looks like a cross between a Transformer and Godzilla's three-headed nemesis, King Ghidorah.
That banner for Transformers: The Last Knight has been been officially released online by Paramount Pictures. You can check it out, below.
— Paramount Pictures (@ParamountUK) August 1, 2016
Said three-headed Transformer appears to be aligned with the Decepticons in The Last Knight (hence the Decepticon flag visible in the background of the above banner), having prompted some fans to speculate that the character might be a Terrorcon, Predacon, or even a member of the race known as the Quintessons (from Transformers cartoon TV series, comic books, and video games past). Other fans have speculated that the creature could be a member of the Dinobots that was not glimpsed during the events of Age of Extinction, though there could be another answer to this riddle altogether.
While the villainous Megatron is (seemingly) coming back in The Last Knight, the overall film is expected to start expanding the live-action Transformers movie franchise mythology, in order to pave the way for it to become a proper cinematic universe. Case in point: Bay has confirmed that the movie will add the Combaticons - a pack of Decepticons led by a fellow known as Onslaught - in addition to brand-new robotic characters like the Autobot "Squeeks". For that reason, there is a fair chance that the Decepticon pictured battling Optimus in the film's banner could be well be one not featured in any of the previous live-action Transformers movies.
Transformers: The Last Knight opens in U.S. theaters on June 23, 2017, followed by the Bumblebee spin-off on June 8, 2018, and Transformers 6 on June 28, 2019.
Source: Paramount Pictures