At this point, I think Light would be the most plausible (and possibly most popular) type suggestion.
But that isn't to say there still aren't plent of existing combinations that haven't really been done.
Like Electric/Fire, Dark/Bug, etc.
But I would certainly like to see Dark/Dragon and Light/Dragon legendaries.Or Light/Steel.
Anyone who suggests Light type is automatically an idiot. Unless it's Light Yagami type. :)
That's a pretty rude thing to say. I don't see how suggesting a "Light" type (which to me is just as plausible as the already existing "Dark" type) would make a person an idiot. --- http://img214.imageshack.us/img214/7388/spectrumf.jpg