Another issue though is the necessity of this. I'm worried about whether we could implement a new type due to variety of Pokemon and whatnot (especially something as narrow as Light-type), but there's also complexity. We're already at 17 types, and that's a lot. We might be able to add in another type, but there's no real reason to. In the least, I'm not arguing that Light would imbalance the game (although that is quite likely), but I am saying that this would add a whole lot of complexity that we don't need.
If it ain't broke, don't fix it. Most of this time we've been arguing whether it could be included or not, but we need to ask why it'd be included. There are no pressing needs to balance anything, nothing's dying for a re-type, and, at least in Light's case, there aren't enough ideas for Pokemon to justify a whole redesign of the element system.
