One other option for Megatron's vehicle mode that would still allow him to fly is an AC-130 Spooky gunship. This plane is ALSO dominated by its guns, including one big fuckoff minigun that basically spits fire. It's nice and big, too, so if you want to keep Megatron a big Decepticon it'll fit the bill.

Honestly, if anything it might have the opposite problem to the A-10: it might be TOO big. If this was Megs' vehicle mode his alt mode might be the size of Devastator. The AC-130's a decent-sized plane.