Not every Transformer must turn into a 2009 vehicle. Heck, it's more likely that some alien landing at random in on earth would bump into something vintage than the latest showroom models and prototypes like we saw in the movie.
LOL A Model T Bot.
Have you ever seen the displays at the Henry Ford Museam? It's not just old Ford cars. It's a massive collection of machines from the last 200 years. Rows of anything from bicycles, to airplanes, to 2 story tall steam locomotives. It would honestly be a great place for a new Cybertronian to get his alt mode. I particularly love the Spirit of St Louis, Ronald Reagan's limo, and the full vintage diner they have there inside the museam.
Make something like this giant locomotive into a Transformer and you'd have one massive Transformers the scale of movie Blackout.