There's literally hundreds of existing Transformers characters who don't have a movie-verse representative. They'd could do fine without reviving any. Of course prequels give you that opportunity to use guys like Megatron if needed.
Leaving aside Movieverse characters that have the same name as G1 characters, you're still left with Autobots like Prowl, Bluestreak, Sunstreaker, Tracks, Trailbreaker, Cliffjumper, Hoist, Red Alert, Gears, Powerglide, Seaspray, Perceptor Springer, Blurr, Ultra Magnus, to name just a few.
And that's without basing characters on things other than G1. Unicron Trilogy's Cyclonus, Tidal Wave, Snowcat, Thunderblast, Thundercracker, Ransack, Crumplezone, Menasor and Scourge are waiting for their turn to cause chaos on the big screen too.
And even teams of Decepticons. You could have movie about a set of Autobot cars (pick any from the list above) on a mission to stop the Stunticons - but maybe I just saw Mad Max and want to see a movie-length transforming car chase.
What I think about when I see all those names is not, what "Transformers" stories are they going to tell, it's what "humans interacting with Transformers" stories are they going to tell. For a good chunk of Budiansky's run on the Marvel comic, those are the stories he told. And while I wouldn't want to see them redo those stories they do show there's plenty of range for human-based films across many genres:
- The romantic comedy about a young woman named Charlene who falls in love with the car she finds broken down by the side of the road.
- The crime film about wannabe gangster Joey Slick who finds a powerful gun that is actually a Decepticon in alt mode (Browning I guess).
- The road trip story about the truck driver Bomber Bill trying to get his truck back after it's stolen by some Decepticons with the help of a talking truck.
- The kids adventure film about a group of Spacehikers.
- The comedy film that follows the misadventures at tourist resort Club 'Con.
- The found footage film about the reporter who stumbles upon a giant monster in the swamps of New Jersey.
- The mockumentary about the making of Monstercon From Mars.
- The antihero film that follows the Roadjammers.
- The superhero film about Circuit Breaker.
I'd expect the Transformers spin off movies to be more inline with those sorts of stories than seeing big screen versions of Last Stand of the Wreckers, Beast Wars, Target:2006, Five Faces of Darkness etc