I'm of the firm belief that Autobots should primarily be land vehicles, and Decepticons should cover everything else. While I'd rather they fleshed out the Autobots they've already got than introduce new characters, I have a feeling we'll see Arcee in a sequel. Perceptor as a communications vehicle would also be pretty cool.
The Decepticons actually need screen presence. Barricade and Frenzy were the only two that seemed like actual characters, and even then just barely. I'd like to see Starscream take charge leading some new Decepticons. Shockwave, Hardtop, and Longarm, all re-imagined of course, would make a pretty decent roster. Give Blackout a little more character, and they might actually be a threat. We'll probably end up with Galvatron, though.