I still find the notion of some of these types absolutely ridiculous. Even though Pokemon has never made the most sense on certain things, I believe that some of these suggested types are just a whole new world of weird.
Starting with Sound, how would that even work? How could sound manifest as a force great enough to do anything? Yes, sound at enough pitch can shatter eardrums and glass, but what would damage it in return? You can't exactly stop sound from travelling unless there's a vacuum, so...a Space type? Um, no. In this scenario, Sound would have to be the ultimate neutral type: can damage everything, and is damaged by everything. No super-effective hits, no resistances, no immunities. And, most likely, largely relegated to a utility role rather than a damage-centric one, in terms of moves.
And "being similar to Water, Fire, and Lightning"? I know it's not outright stated in the games or the anime, but Water-types don't just carry water around with them, their bodies have some mechanism that's able to convert hydrogen and oxygen from the air into water, and then pressurize it into a burst. Fire attacks are, as are a common tradition with dragons irl, have special glands that can ignite a spark, that with enough of an exhale, can send the spark out to ignite the oxygen. The Pokedex entries of most Electric-types involve the gathering of storm clouds, which suggests that the Pokemon themselves act to charge the ions/electrons around them while the stormclouds gather, and then turn themselves into a lightning rod of sorts. Being elemental creatures the power of which is unknown to man, they are able to direct this lightning through force of will. Another means would be a static discharge.
It's not 100% real world logic, but it's also a fantasy game; however, it is a rather grounded fantasy game. Pokemon are stated to be creatures that are beyond what we know creatures to be on Earth, with some being outright called deities.
In regards to Alex's picture about Light/Sound and the definitions thereof, wouldn't that essentially make all Electric-types Light? It only stands to reason that any and all Electric-types are able to give off light in some form or fashion, even if it's just a glow from being "charged".
And then there's the metagame to consider. While this has been brought up several times in similar topics, Dark and Steel exist almost solely to stop the unholy rampage that was Psychic and Dragon, respectively. They also weren't brought about to retrofit old Pokemon, as there are no Dark-types in Gen I, and the only two Steel-types are /magnets/. Bite was a Normal-type move to start, and that hardly seems like a Dark-type move. It was shifted so that Dark actually had some moves. Gust was also a Normal move, and was retrofitted to Flying because it made more sense. You can argue that with Dark moves mainly being dirty tactics that Bite would fit in there, and it kinda is, but with most Pokemon being animals, biting is a natural form of defense and offense, so it would technically make more sense as a Normal type move since it would be so commonplace. Although, I can see that from the perspective of GameFreak not wanting to provoke PETA, that Bite would be shifted to avoid "dogfighting"(even though a ton of Pokemon still learn it anyway >>).
So that brings us to the latest victim of internet fanboyism: Fairy-type. One word: Navi. Welcome to our new mascot. In all seriousness, we have several pixie-like, or "fairy-like" Pokemon, namely legendaries, most of which are Normal-type or Psychic-type. This is because they have special powers related to the metaphysical. Let's take a look at Celebi. It's Grass/Psychic. Now if we change that to Grass/Fairy or Psychic/Fairy, both times it seems almost redundant.
While the same could be said for Rock/Ground, let it be known that while birds will not suffer to an earthquake, we do have the phrase "two birds with one STONE". What this basically means is that Ground moves deal with things on the ground, while Rock moves deal with things off of the ground. So while similar, they are different. But what about Water and Ice, you ask? Well, you can't exactly karate chop water and it do much, can you? But breaking ice is a common feat of strength. But why is Ice the only non-Dragon type to be Super-effective against Dragon? Dragons are typically reptiles or close to them, no? And what are reptiles? Cold-blooded. Heat they can withstand because they can naturally produce it and they need it. Cold temperatures would severely mess with their internal structures by making them lethargic and unable to do much to fight back against it. That's why most Dragon Pokemon are FOUR TIMES WEAK to Ice. They are so crippled by it that it doesn't even matter.
In terms of what a new type would do to the metagame, let's look at this: Poison is only strong against one type. And yet, Toxic and Toxic Spikes two very widely used moves in the metagame. This is because they offer great utility, and yet can easily be stopped with a commonplace Steel-type or another Poison-type. The metagame is essentially balanced as-is. If you have good team synergy, you'll probably win, regardless of if you're using a weather team, gimmick team(see: Tailwind, Gravity, Trick Room), or just a typical type-balanced team.
What I'm trying to get at is this: a new type would just upset the balance. There is no overpowered type because there are so many dual-type combinations now that just about every Pokemon has at least two or three hard counters, and most Pokemon have a decent enough range of usable attack types to get at worst, neutral coverage on everything, with a good number of Pokemon getting better than that.
And if Nintendo is going to call a Mewtwo Forme a "new Pokemon", then it's quite possible that they are making Sylveon's type secret just to have us hyped up so we'll buy the games. Additionally, with it being an international release, they'd obviously want to tell us as little as possible so that we can be genuinely surprised(and buy the game). I'm not saying they're in it just for the money, but they know that this level of secrecy is going to make us buy it so we can find out. Everyone and their mother has wanted a new Eeveelution: well, we got one. If you're happy with it, fine; if you're not, also fine. A lot of people were angry about Emboar being Fire/Fighting, and that's with existing types. How easily could a new type receive such a reaction? Very. It has to be executed nearly flawlessly, so it's either really carefully planned out, or it doesn't exist.