What do you think he'd have been? Assume he'd gotten to scan something. Looking at his cybertronian form, he can fly like Starscream and he seemed fond of doing so(what, with taking Prime thru a building and all), so he'd probably have picked something capable of flight and like the other Decepticons, probably a military vehicle. So going by that, I was thinking about what kind of aircraft would fit his size. Megatron is supposedly slightly larger than Starscream, correct? So the F22 (65 feet) wouldn't fit as well. So that also rules out the F-15 and probably the F-14 (especially since it was phased out by filming). The F-111 would be roughly his size, but it too was long in retirement. The B-1B is much too big. The B-2 has roughly the same size problem as the B-1, being 172 feet wide. Other consideration, the eurofighter Typhoon, but it's likely too small at only 52 feet long.
The only thing I could come up with that's bigger then the F-22 and would fit the role was the SU-27/30 or SU-33. It's 7 feet longer or so, weighs more, and is still in active service the world over.


The cool thing about the SU-33 is that it's the carrier based version of the SU-27/30, so if there was a decepticon hiding as a carrier out there, it'd be at home on deck.
Down side to both is that they virtually don't exist in America, except for maybe an international airshow exhibit or such. So if he was here, it wouldn't be a good disguise.
So what would your image of his alt mode be?