Considering that Transformers: Age of Extinction was the highest-grossing movie worldwide for 2014 (in spite of bad reviews), it stands to reason that a sequel would be made. Originally, it was thought that Transformers 5 would be released in 2016, but it looks like there will be a delay as Hasbro is still in the planning stage for the film.
A most recent report from IGN states that Hasbro CEO Brian Goldner has shared an update on the Transformers franchise, as well as a potential release date for the next film in the series, Transformers 5 or whatever its subtitle will be.
The statement was made during Hasbro's 2015 Q1 Earnings Conference Call. Goldner stated that the sequel to Transformers: Age of Extinction will be happening in 2017. The producers and Paramount apparently wanted a 2016 Transformers film, and apparently wanted to have a new film come out every two years. Considering all of the delays of information regarding Transformers 5, I am not surprised.
As for the plot for the film, Goldner announced that the studio and filmmakers hired some writers including Akiva Goldsman to create a strategic plan around the Transformers franchise. If you aren't familiar with Goldsman work, he has written many speculative fiction movies including two Batman films (Batman Forever and the now infamous Batman and Robin), Lost in Space, I, Robot, and won an Oscar and Golden Globe for his work on A Beautiful Mind screenplay.
Goldsman is one of the people who Hasbro wants to join their "writer's room"with Michael Bay, director of the first four films. Also included in the "writer's room" are producer Lorenzo di Bonaventura and executive producer Steven Spielberg, and it will be a host site for potential sequel ideas, not to mention possible spinoff films.
So what kind of spinoff plans are happening? There are still rumors going on about bringing aboard Bruce Willis and Dwayne Johnson and creating a G.I. Joe and Transformers crossover film. Considering that Hasbro owns the rights to both franchises, it could be a potential box-office draw. However, Lorenzo di Bonaventura, the producer of the Transformers, has stated in an interview that this crossover was "not a realistic proposition, at least not in the near future".
We know for sure that Michael Bay is not going to be taking the director's helm here, this means that someone else will have to take up the reigns. It will probably be the director of Teenage Mutant Ninja Turtles, Jonathan Liebesman. As for what Michael Bay is doing, he plans to work on a Benghazi-based drama 13 Hours, which has no planned release date.
This leads to fans wondering what Transformers 5 could possibly be about. At the end of Transformers 4, when Optimus Prime blasts off into space, is meant to be a set-up for Prime to arrive on Cybertron in Transformers 5. If so, then he'll probably go after the creators of the Autobots and Decepticons, who are the 5-faced Quintessons from the animated series. This could lead to some very interesting computer-generated graphics, but not much human interaction, and there has been little word on the cast as yet.