Pokemon Grey will be just like Pokemon Platinum, Pokemon Yellow, Pokemon Emerald, and Pokemon Crystal. Same game with the trainer having slightly different clothing, a few new areas that looks to be simple DLC and fashioned as a new game.
Pokemon B2 and W2 are direct sequels. We don't know what to expect as this has never happened before in terms of pattern of games. Hopefully we don't see a trainer picking his first Pokemon and setting for a journey because I think GameFreak beat that dead horse a long time ago. There are a ton of stories to investigate after all.
Pokemon Grey is absolutely not necessary. Asking for Pokemon Grey is like asking for black and White to be remade with very few changes. B2 and W2 have the potential to be very different. --- Too many Games, soo little time. Add me online: Xbox Live / PSN: levyjl1988 | 3DS FC: 4639-8984-4691