I'm not sure where this was first mentioned, but TFW are saying that it has come from Paramount... and even though character-based movies were going to be the next phase (like with Star Wars, and if Bumblebee is a reboot it could be like the Marvel Universe, starting with movies that focus more on one character before bringing the band (back) together), it would make sense that the other main identity/face of the Brand would be the title of the next one.
The other notable bit of info is that Travis Knight hasn't been called up to do another Transformers Movie at this time, but it is still very early - the Bumblebee Movie hasn't even been officially released yet.