The rail gun that shot Devastator? Movie TFs turning into guns isn't unheard of, since Mixmaster did it in the movie.
Shockwave in any capacity would be AWESOME!!!!! Maybe if they are planning on going past a trilogy for the franchise at some point in TF3 they could show him calling Megatron from Cybertron pleading for help or something to that tune?
I hope Megs returns to the Con HQ and finds Shockwave, who thoroughly beats down the damaged and weak Megatron to take Decepticon command. But that's just my own personal hope for TF3.