After reading a couple of pages of everyone's opinions, I feel like I should add some input into this topic. Ok, here we go... *cracks knuckles*
Based on a standpoint of Game Freak and Nintendo, I am pretty confident that a new typing would become a necessary addition to the other 17 types. Why? Well.. Sooner or later, Pokemon will lose its versatility in type combinations and fans will begin to get sick of the repetitive type groupings in the game. People WILL demand something new and different from Game Freak, and you know they will. Like every other entertainment/video game franchise, changing into something new and fresh keeps the company alive and thriving. If we keep the same types for much longer, fans will die out and that is something no company would want to experience, and I'm sure of it. I'm not saying that this problem is happening now, (except for fire/fighting starters. Oh god Game Freak...Please don't let us down with Fennekin ;__; )we still have many interesting type combinations left and still waiting to be revealed. But, like I said before, it WILL happen. Who knows? Starting now could be a safe move for Game Freak. It's a fresh, new generation, so why not add something like a new type as well?
Personally, I am absolutely happy and fine with the types we have now and wish it would never change, but sadly, everyone would agree that at one point in the future that would have to change. Even though this may seem upsetting to many people (including me), it might be a force of action that would have to taken by the company for the sake of Pokemon. When you think about it, one type wouldn't hurt so bad. You and I might even like it, but that we will just have to see. But hey, if this is the decision they make, then it's the decision that is made. To me, Pokemon is Pokemon, and I will always be a loyal fan for the anime/game. ^.^ We will just have to trust Game Freak/Nintendo in creating a new type IF that is the path taken. Hopefully, it would be a fresh, fun type AND one that would be balanced and equally accepted with the others! I think a light type couldn't be THAT bad of an addition. After all, it's pretty logical since all types have an opposite like flying/ground, fire/water, normal/ghost. I just hope that whatever decision Game Freak makes, it would be a great one! Pokemon forever! ^^v
TL;DR: A new type could be necessary in the future because of the possibility of type combinations to run out. Personally, I hope not.
Oh, and just a sidenote, im on the same boat as everyone else with the whole balance situation because I really do believe it would most likely be too much. But I thought since everyone was having the same idea, it would be nice to add in something different that could be considered as well. I hope this would not happen!! >.< I'll stop talking now...hehe :)