The humans themselves wouldn't be wearing rags like a bunch of peasants from an episode of Xena or anything. Since this is 300 years after the TFs left Earth, they've had time to rebuild their civilization, and while it wouldn't be close to on par with our current world, they'd have good clothes and basic technology. I wouldn't want it to be all doom and gloom.
I have this idea for a scene in which K-9 infiltrates a human camp disguised as a dog, and I like it.

There'd still be wreckage of all sorts of vehicles littered about, so if you wanted to include some G2 bots like BW2 did, it'd be easy enough to work them in by scanning a junked tank or jet.
I imagine areas where the vegetation is lush and green and dominating the land, and other areas where cities are still veiled in a fog of invisible cybertronian radiation from the fallout of TF weapons 300 years prior. Easy way to work in some fuzors with the radiation possibly screwing with scanning a beast mode.
The basic theme of the story/show would be about having to deal with the consequences and the aftermath of your actions. I figured it'd be a nice theme since BW was sort of about the past and pre-destiny and setting things in motion, my story would be set at the other end of the spectrum. Anyway, that's all I got.