There's a thematic reason why Game Freak never made a long-rumored Pokemon Grey series. In a recent interview with Game Informer, Junichi Masuda explained why Game Freak opted for Pokemon Black 2 and White 2 over a standalone sequel like in past generations. "A lot of people were expecting Pokemon Gray at the time, but the concept of Black and White was kind of these opposing forces – a yin-yang kind of thing. If we went with Gray, it would have moved away from that concept so we decided to keep the titles there."
Pokemon Black 2 and White 2 marked a major departure for the Pokemon franchise, as Game Freak had previously opted to make a "solitary game" when following up with one of their core pairs of games. Pokemon Red and Blue had Pokemon Yellow, Pokemon Gold and Silver had Crystal, Pokemon Ruby and Sapphire had Emerald, and Pokemon Diamond and Pearl had Platinum.
But Pokemon Black and 2 was the first time Game Freak had made a direct sequel within a single generation. Although not the first sequels (Pokemon Gold and Silver hold that distinction), Black 2 and White 2 were the first games to break away from making a "trilogy" of games, a tradition that the Pokemon franchise has never returned to. After making no follow-up games for Pokemon X and Y, Game Freak is about to release Pokemon Ultra Sun and Ultra Moon, an "alternate storyline" from the original Pokemon Sun and Moon games.
In the same interview, Masuda also indicated that there were no plans to stop releasing remakes of older Pokemon games. "I don't think we're focused exclusively on making all new stuff," Masuda said. "I think it's important – the nostalgia factor is one thing – to give these memories, this content, another chance to shine for new players. And as a player myself I enjoy that kind of thing, as well."
Remakes of older Pokemon games have become somewhat of a tradition for the franchise, beginning with the release of Pokemon FireRed and LeafGreen nearly 13 years ago. Players are now hoping for a remake of Pokemon Diamond and Pearl, which has been fueled by a recent re-emphasis of Gen 4 Pokemon in merchandise and the latest movie.