I've seen a lot of people who want Bumblebee to be the beat up '70s Camaro from the 2007 movie in this spinoff, but I don't think he should/will be. It was implied that he took on that disguise to appeal to Sam Witwicky (so he would buy Bee at the car lot), but Sam wouldn't be around in the '80s. If anything he should be an '80s Camaro, or a different car entirely.
I don't think the movie should end with Bee pulling into the car lot from the 2007 film. That would be a really awkward time jump, and I think this movie should distance itself from the Bay movies as much as possible. Just make it a standalone story.
Hailee Steinfeld's character shouldn't be a Witwicky. Not everyone has to be related in movies, and I like the idea that Transformers have adventures with random people. Plus, Shia LaBeouf is the last person you want to bring back memories of, lol.
Just thinking out loud, these are some theories I've seen going around the Internet that I thought I would address while we're waiting for some more news to come in.
the 1980s? He's a VW bug again?
so how does this square with the continuity of the previous movies?
Well, these movies have never been known for their strong continuity, but I don't think either of these facts contradict anything in the existing movies.
The movies aren't tied to the original cartoon, so technically he's never been a VW Beetle in this canon. (Actually he was a militarized VW Kubelwagen in the TLK flashback, but it wasn't the same model as the traditional VW Beetle.)
The Last Knight established that Bee has been on Earth since the 40s (at least), so I suppose he was just hanging out, having adventures and looking for the Allspark between then and 2007.