Alright, as the third game speculation has begun, I think I shall present you all with my speculation of Pokémon Gray, Pokémon Grey, ポケットモンスターグレー(Poketto Monsutā Gurē).
First things first... we all know that most of the main Pokémon games are released in Japan in September, right? Well we know that some of the games like Pokémon Platinum only was released 2 years after Pokémon Diamond and Pokémon Pearl, however... that pattern doesn't always follow in the same way. It could either be... that Pokémon Grey/Gray would be released in September 2011 or that two new Pokémon Mystery Dungeon games would be released using full 3D dungeons with one for the Nintendo DS as DSi Enhanced and a 3DS exclusive one (alot like what it was like for Pokémon Mystery Dungeon Red Rescue Team and Blue Rescue Team).
However... if Pokémon Grey/Gray was to be released... it would have to be DSi Exclusive. Why? Because the Nintendo 3DS would already be released, however... they cannot jump to another platform with a third game... because it wouldn't match in the same way. Just think how Pokémon Gold and Pokémon Silver were released for GameBoy with GameBoy Color enhancements and then Pokémon Crystal was released as a GameBoy Color exclusive. It could be likely that Black, White and Grey/Gray would follow in the same direction as generation two. The Nintendo DS and Nintendo DSi are almost mirrored versions of the GameBoy and GameBoy Color. Because the GameBoy and GameBoy Color almost have the same engines to each other, it's almost the same thing that happened with the Nintendo DS and Nintendo DSi. So just think... Pokémon Gray/Grey could well be... a Nintendo DSi exclusive, which means it'll only work on the Nintendo DSi or Nintendo 3DS. And considering the fact that there's only a very few games that are currantly DSi exclusive, yet I'm pretty sure Nintendo will release alot more after the Nintendo 3DS has been released. It's been a long time since the release of the Nintendo DS Lite, correct? The Nintendo DS Lite will begin it's phasing out at some time.
There is a reason why I thought of Pokémon Grey/Gray, and same with everyone else... because every main Pokémon game is based off the main legendary Pokémon of the game. However, the only generation that was not based off the legendaries... was Pokémon Red, Pokémon Blue and Pokémon Yellow. As they were based off the starter Pokémon. This is not likely to happen in the third game of generation 5 and I'll tell you why, because... first of all, Pokémon Black has Reshiram, then Pokémon White has Zekrom. Now just think... the third installation of generation 5... would have to be a legendary, correct? Well... we've got a Pokémon that has the same battle music as Reshiram and Zekrom, it's none other than... Kyurem! Because this Pokémon is grey, it just shows what the third installment will be called. This legendary Pokémon also looks similar to the two as well, it's the Snow Gray Pokémon! It is able to use Ice type attacks as well, there were two moves that were not usable in Pokémon Black and Pokémon White, as these moves were known as Cold Flare and Freeze Bolt. These moves could be the signature moves for Kyurem in Pokémon Grey/Gray.
Alright, as for the storyline... I probably might expect what could be in store for the third installment. First of all... it's all about choosing your starter Pokémon, just like in Pokémon Black and Pokémon White. In the first gym... the same battle happens. You face the gym leader that has the final Pokémon that your starter is weak to. After that, you continue on with the same storyline until when you get to Iris and Shaga. In the city(Souryou City or whatever it's called) where the past and future designs are used for whatever version you're playing, you get to see that city in the present style rather than an alternate timeline. And instead... Iris and Shaga will face you in a double battle in the gym rather than just facing one. Then you face the Pokémon League and getting to the top when after N summons his castle (which would look grey, just like it looked different between Pokémon Black and Pokémon White), and just before you go right to the stairs... the whole mountain gets covered in a snowstorm as suddenly Kyurem appears! As you would've actually seen Kyurem many times during your adventure in Pokémon Grey/Gray and wherever it started snowing heavily. You face Kyurem before you go into the castle and capture it. Then once you get to the same room in the castle where you battle N in Pokémon Black and Pokémon White, you'll see the cutscene where both Reshiram and Zekrom come into the castle and you end up battling N with Reshiram and Zekrom on his side. Or maybe... he might battle you with Reshiram, then you face Getis (Team Plasma's Leader) with his Zekrom.
As for the post game features... I'm pretty sure the area where Black City and White Forest is in... would probably be known as "Mt. Grey/Gray", as you'd have a cave to going into as well... which would bring you into a cave known as "Grey/Gray Cave". Oh and not to also mention... that somewhere in the Unova region... would probably be the Battle Frontier. Which would be in the same place as wherever the Battle Tower would be in Pokémon Black and Pokémon White (as I'm not sure if there is one). As for Zoroark and Zorua would be obtained in the same way as Pokémon Black and Pokémon White.
It could also be a possibility that some Pokémon might have new formes in Pokémon Grey/Gray.
So I guess we should keep a lookout for it to be announced on the Japanese Pokémon website once it does. There's been huuuuuuuuuge speculation about Pokémon Grey/Gray over the internet.