As noted here, a preview was put on the CoroCoro site, revealing the contents of next month's issue. According to it, a new Pokémon game (currently referred to as "the new game of mystery") is to be announced next month. Furthermore, the magazine will commence the "Path to Diamond/Pearl" project.
It should be noted that the previous issue of CoroCoro stated that other than Diamond/Pearl, two or more games are scheduled to be released. As a reminder, the said issue included an interview with Gamefreak's representatives on the subject of Diamond/Pearl, as well as the announcement of the Pokémon Ranger project. Judging from the description, it would appear appear that Pokémon Ranger is a Nintendo DS game, as well as the title of the next anime series and movie. The entire project is supposed to precede the release of Diamond/Pearl, hence the title "Path to Diamond/Pearl". If to judge based on the name alone, Pokémon Ranger strikes one as a spin-off game. It is hard to imagine how this could represent Diamond/Pearl, or how it could later be used as the title for the anime series. Either way, it is safe to assume that the Pokémon Ranger game will not fall under the category of RPG games, seeing as it would be absurd to have a separate RPG game that would be meant to be followed by Diamond/Pearl. Needless to say, this raises many questions that will hopefully be answered next month.
Since the magazine is said to unveil a new game and offer news on the project, it is likely that the former is not Pokémon Ranger. This game is likely unrelated to Diamond/Pearl. Although it is also said to be one of the upcoming Nintendo DS Pokémon games, I believe that it is merely a generalization, and that the game will in fact be for the Gameboy Advance.
At the beginning of the year, Tsunekazu Ishihara and Jun'ichi Masuda stated in an interview that there would be a new title for the Gamecube (now known to be XD), new titles for the Nintendo DS (Diamond/Pearl) and a new title for the Gameboy Advance, all of which would be the focus of attention in 2005. Many did not regard importance to the mention of a new Gameboy Advance title, assuming that it was a reference to Emerald (which was illogical, as by that time the game had been in released in Japan and had been the focus of attention for the second half of 2004). With signs of another upcoming game, I am inclined to think this is the Gameboy Advance title which was brought up in the interview.
The release of the game should coincide with that of the Gameboy Advance Micro system. Nintendo has stressed many times that the Nintendo DS is not to replace the Gameboy Advance series, and yet the company's source of profit is mainly attributed to the Nintendo DS. The release of the Gameboy Advance Micro system is specfically addressed at raising the sales of the entire series, as if to prove that the Nintendo DS has yet to completely replace its place. It seems reasonable to release a new Pokémon game for that goal alone.
I suspect that the game will be the last addition to the third generation of RPG Pokémon games--the third game to FireRed/LeafGreen. I have seen the possiblity raised many times, concluding that most fans view it as far less than desirable. Those who have shown interest in such a game, have only discussed whether it should be the remake of Blue or Yellow. I have yet to see a discussion on what new aspects the game could contain that would make it desirable; people have dismissed the notion without examining the unused data in the current games that could be put to use through such a game. Suffice to say, I believe that there is much to be said on the matter, especially now that there are signs of what is to come.