Re: Pokemon Black and White
Honestly adding a Light type would not be that hard at all. And I'm saying this even though I STRONGLY believe that they are NOT going to add them.
I would guess that a lot of protection-type moves would become light-type, such as Reflect and Light Screen (both of their descriptions say they create a barrier of light) as well as possibly Heal Bell and Safeguard, and maybe Morning Sun+Moonlight. Helping Hand is also a Light-ish move.. I'd imagine a lot of the protection/help teammates/happyjoyjoy-themed moves could make the switch.
New attacking moves could be made easily (I'm assuming they're adding new moves anyways, and Dark and Steel didn't have many moves when they were first released anyways). There are already some that sound light-type, like Luster Purge, but I don't think they'd want to mess up it being a STAB for Latios, unless the Lati twins switched to Light/Dragon instead of Psychic/Dragon, but I don't believe they would change legendaries like that.
Perhaps some of the cute fairy egg group Pokemon would make the type switch, like Blissey and maybe the Togepi-line who are always described as being incredibly helpful and good-natured.
I'd assume Light would take the place of normal for Togepi/Togetic/Togekiss and it might be a dual-type for Blissey (Normal/Light).
Other candidates include the Gardevoir line (Gardevoir and its preevos are always described as being very caring and emotionally sensitive to their trainer, Gardevoir even being willing to sacrifice its life for him/her)..
That being said, I do NOT think that Light will become a type, and I don't really want it to, either. Psychic sort of fits that niche right now with it sort of representing "Powers used for good" or w/e with Dark being, well, Dark. Psychic is Light and is also Neutral.
Also, what does a "Light" type look like? Ethereal (like Ghost?), angelic? Is it just made with a lot of white coloration? I think if there were a lot of them we'd get really bored of it, as it's a concept that doesn't seem to lend itself as much to creativity like the other types.. And the concept isn't that mixable. Like, I can't imagine a Light/Rock Pokemon, would it be a giant scary rock angel? Or Light/Grass, the angel plant! Unless the grass Pokemon was highly anthropomorphic like Roserade, I can't see that happening.