Hm...not sure what you could do for the combiner head, but maybe you could use Cyclonus as a center/leader. He already has the swept-forward wings of the jet-mode Vehicons from TFP, maybe paint him silver to make him one of Starscream's Vehicon generals?
I think it would look better as Cyclonus painted to resemble a Vehicon jet general as someone pointed above. Also, great job, but only one question... Who's Steve?
Instead of Cyclonus/Megatron/etc. Why not take a Silverbolt and turn it into Prime Arachnid. Considering her vampire plague thing at the end of the series, it'd make sense for her to combine with vehicons kinda. Additionally, you could have her four helicopter blades as folderable guns. Unfolder and plug-in for helicopter mode, unplug&fold to become rifles for the four vehicons.