I'm in agreement with the others, a new type would throw things way outta whack. Especially considering that they would need to make some older Pokemon Light-type to populate the typing, or make a bunch of the new ones Light type which would throw off the balance for the new generation. Unless it would be used to fix some glaring hole like what happened with the Psychic types (and I can't see any hole in the current set; even the all-powerful Dragon types are weak against other dragons and ice, and can't do much damage to Steels) I don't see the point.
From where I see it the 17 types work together quite gracefully to make a system I have come to deem "perfectly imbalanced". A perfectly balanced system has all its parts completely even with the other parts, but as a result is kinda bland and uninteresting on the meta level. Think water-earth-fire-air. But the way Pokemon types are set up, there are some that have certain advantages, like Dragon types having access to extremely strong moves and Steel types having a lot of resistances. These slight imbalances cause certain strategies to be developed among trainers, which cause OTHER trainers to use strategies that counter and work against the first trainer's strategy due to some critical weakness in his typing (like ice and fighting); and so on and so forth until it gets to the point where any one strategy may be entirely advantageous or disadvantageous depending on what the other person does, with no one strategy coming out as a clear winner above all others. A system in perfect imbalance.
And besides that, there are still a lot of dual-typings that have yet to happen, like a fire-water type or a dark-psychic type. I'd like to see even more experimentation on that; Golurk, Volcarona, Galvantula, and Hydreigon are among my favorites now precisely because of the wacky typings.