From a Hasbro press-release, from January 23rd, Transformers fans will be able to vote for the new face of Transformers Generations, for the third series of the Prime Trilogy, so far being called "Power of the Primes".

The vote will take place through the Transformers.com website, but since it region-deflects non-Americans to their own country's website, it is not yet known if non-Americans will be able to take part.

Voting must only be for a few days too, as the second round of voting from a short-list of four finalists will take place from February 1st to February 4th.

From what I can work out in the press-release and promotional image, there are nine candidates who are part of three groups - Hono(u)r, Chaos, and Order. From those nine, the first round of voting will reduce it to four, and the second round of voting will give us a winner that will be announced at the New York Toyfair in February.

The nine candidates are Ultra Magnus, Hound, Arcee, Shockwave, Megatron, Star Saber, Thunderwing, Unknown Evil, Optimus Primal.

HASBRO LETS FANS SHAPE THE FUTURE OF THE TRANSFORMERS FRANCHISE WITH THE “POWER OF THE PRIMES” FAN VOTE
The fans can decide which Transformers character will become the next leader of Cybertron!
PAWTUCKET, R.I., December 1, 2016 – Today Hasbro, Inc. (NASDAQ:HAS), announced the “Power of the Primes” fan vote, an upcoming event that will place the fate of Cybertron directly in the hands of Transformers fans around the globe. Beginning on January 23rd, 2017, fans will be able to visit Transformers.com to cast their vote to help determine which character will join the ranks of Optimus Prime, Rodimus Prime and other Cybertronian legends. The fan-voted Prime will be featured in the Transformers Generations toy line and brand’s Prime Wars Trilogy storytelling in 2018.
The Prime serves as leader of Cybertron, wields the immense power of the Matrix of Leadership and holds the fate of the Transformers universe in their hands. This leader is usually chosen directly by the Matrix of Leadership, but the “Power of the Primes” fan vote will put the fans in command and give them the power to light the darkest hour and choose the bot to lead Cybertron into a historic new era.
“Hasbro’s past Transformers fan votes have led to some of our most exciting characters, including Windblade, Victorion and next year’s Trypticon Titan,” said Tom Warner, Senior Vice President of the Transformers franchise at Hasbro. Our “Power of the Primes” fan vote will raise the stakes higher than ever. The Prime sets the tone for all things in the Transformers universe, and we’re excited to see which direction our fans want to take.”
The first round of voting will begin on January 23rd, when fans can visit Transformers.com to choose between three groups of candidates – Honor, Chaos and Order.
Honor: These legendary warriors will lead with honor and carry on the noble legacy of Optimus Prime.
Chaos: These powerful warlords will stop at nothing to become supreme leader of Cybertron, even if it means creating a chaotic world where only the strongest survive.
Order: These commanders vow to leave the Autobot and Decepticon factions in the past and unite Cybertron under a new order.
Four finalists will be chosen by the fans and revealed by Hasbro in late January. Fans can then have the final say by returning to Transformers.com to vote between February 1st and February 4th. The winning Prime will be officially unveiled during New York Toy Fair in February.