I haven't seen this posted here yet, so I thought I'd post.
A Cab Over Engine (flat nosed) semi truck was spotted on set.
It was posted on Instagram with the caption:
"One of the new #transformers from yesterday movie shoot #dinobots, who are u!!! Damn sexy a.f that's what'd up #michaelbay #markymark".
Now some think this implies that it is in fact a Dinobot, but that's not possible for a number of reasons, including that they are Dinosaurs in their "vehicle" form.
My guess on who it could be is:
A) Ultra Magnus (the most likely, IMO)
C) Optimus in a form he takes at some point in the film.