The serum thing had always been part of the Goblin's story, in all iterations. Ultimate merely disagrees in the extent of its effects. I say, do a Gremlin-like face on the guy, but leave him looking scrawny. That should distinguish him from Hulk, and bring actual Goblins to mind. I agree though, that without connections to the larger Marvel universe wherein Cap, Hulk, and GG are all results of research into the same concept, it loses some of its plausibility.