I don't really want the game to have a new type, I think that there is a pretty good amount as it is. They all mix fairly well, although I wouldn't mind it if they made some of the types more usable by giving them a little boost in the next generation, but that won't happen. As someone said about Palkia, it never looked like a Water-type, so appearances are usually not very helpful. It took me longer than it should have to realise that Palkia was a Water/Dragon type pokemon. On that alone I doubt that there will be a new type in the game.
Why would a Light type break the game? They could make it work with alittle focus and testing.
Light
Strong vs
Dark,Ice,Light,Ghost and Water
Weak vs
Light,Normal,Steel,Ground and Rock
If I was going to introduce a Light-type into the game, I'd make it strong and weak against the Dark-type and not itself. I don't really see why it would be strong and weak against itself but then again I never saw the connection with the Ghost-type being like that. I think that it could fit into the game and not break it but it's very unlikely that they would find the right balance. As Xinnamin said, they would flood the next gen with a large amount of Light-types which would make the game boring and would probably break the entire competitive side as a whole.
I agree that it would be a bad idea overall, but given how Steel and Dark moves weren't insanely prevalent in Gen II (and given how Dark moves STILL aren't very prevalent), I doubt an over-representation of any kind would happen.
This is a good counter to Xinnamin's argument though, but that suggests that there would be a rather large lack of them and as it's take so long to get the amount of Dark-types there are now, it would take forever to get a decent amount of Light-types.