Now the days when this would have been the coolest crossover on the block are long gone. But I think it could still work.
In a number of ways, Spawn was derived from Venom; appearance, using a living costume, brutal methods. It could be done where Venom and Spawn run into each other, interact, fight, and then start to argue over who's the original. There could be a lot of fourth wall breaking, and real people like David Michelinie, Todd McFarlane, and Stan Lee could be actual characters in the story. They finally go to court with Marvel and Image comics representing their respective characters. During the trial, one of the witnesses called to the stand to testify for Venom is Nightwatch (the two have worked together):
Spawn of course goes ballistic seeing this guy, and a massive fight ensues. Eventually, they all team up to fight the Violator bonded with the Carnage symbiote, or something like that. I think if done well, this could be a really clever combination of humor and action. Any takers?