It's just a bit too late in the series to start adding new types, if you ask me. It's not impossible or the worst idea in the world, just something that'd generally shake up the consistency of Pokemon as a whole.
Now, reordering the type chart could still be done and quite nicely at that. (Poison needs more SE's.)
I honestly would like to see a "Light type" be introduced. Light has nothing to do with electricity, or Fire for that matter. its a Radiant or Holy element that only the purest of pokemon should know. I'm referring to the fairy egg group like Togepi, Clefariy and Chansey. Maybe even Sylveon and the new legendary Xerneas could be light types as well Simliar to Dark it should have its own type i mean there are already moves in the game that can be light such as flash canon, calm mind and light screen, and i mean if they can change Bite from a normal type to dark i'm pretty sure they change curtain moves to light. Light types would also have some type of weak/ strength against Dark types. Now i know before anyone says it Fighting types are suppose to be blah blah blah, that boring in my opinion Dark types are said to be "evil pokemon" who draw their powers from evil though and cheating ways, well to me the opposite of that doesn't involve fighting for honor and glory... its drawing powers from good and pure though in other words Light! and it would also bring the types chart to an even 18.
I don't see how adding a new type would benefit the game.
Magic can be put with Psychic Light can be either put into Fire and Electric. Plasma can be a duel Type Fire/Electric pokemon. Fairy are magic beings, refer to magic. Radiation can be put as Electric Type.
Then why are rock and ground to different types If they did this then I can see them making a light, magic, sound, or whatever type