I'm in the camp that thinks that the X and Y will refer to both Genetics AND dimensional space, since both seem to have a lot of evidence supporting it and there's simply no reason NOT to have both.
As for the whole "light type" hoopla, I'm calling bullshit on it (like most people). I think the only reason that Gamefreak introduced new types in Generation II was because the original type table was flat out imbalanced, and they needed to fix it. The type table itself is actually fairly balanced now, so I very much doubt they'd risk shaking it up with some bullshit "light" type. Not to mention how badly a light type would screw with the popular and stable fighting > Dark > Psychic triangle, simply because how can you NOT have Light type be directly related to Dark?
No I think that a new type is both unlikely and unwarranted. Better off with what we have now. It's not like we've run out of possible type combinations yet or anything.