Today Paramount announced the planned release dates for the next Transformers films.
Transformers 5 - June 23, 2017
Bumblebee Movie - June 8, 2018
Transformers 6 - June 28, 2019
Michael Bay is set to direct Transformers 5 with Mark Wahlberg as the star. To maintain that ambitious release schedule, where each film needs about 18 to 24 months each to make, Paramount has pretty much guaranteed Bay will not be directing Transformers 6 (but nothing stopping him from doing TF 7) as Bay seems to like a year break from the TF franchise. This may explain why Transformers: Robots in Disguise is ending after only two seasons. At this point the cartoons exists to fill in the gap between movies and there will be no gap for three years (at least).