I haven't decided which side I'd pick yet. Autobots are cheesily good and decepticons cheesily evil, but that's just the way it's supposed to be. Decepticons tend to transform into slightly cooler things, but the whole vision of the universe is geared toward rooting for the Autobots, so playing Decepticon might go against the intended concept and make it feel less Transformers-like.
As to what I would transform into, that's a difficult question on it's own. Many Decepticons go for flying craft, which might be fun, but since all transformers seem to be able to fly (at least int the animated series), it might be more fun to be a land vehicle from time to time, which would most likely mean going Autobot.
For weaponry, I'd like to have the standard beam guns they use and some solid melee skills. I wouldn't want to be as big as Optimus Prime or anything, though, so I might be at a disadvantage in melee, depending on how the classes would be built.
The special tricks I would be willing to forgo in favor of better usual combat skills, as I don't have any huge ideas as it comes to them. Maybe a grappling hook for catching elusive foes and towing damaged allies?
SWG: CL90 Smuggler, Crimson Phoenix Ace, CL90 Engineer, Europe-Chimaera
AoC: L70 Guardian