For a long time before the announcement of Pokémon HeartGold and SoulSilver Versions, it was believed by many that remakes of Pokémon Gold and Silver would be produced and released during Generation IV on the Nintendo DS.
The possibility of these remakes took their precedent from the release of Pokémon FireRed and LeafGreen, Generation III versions of the Generation I Pokémon Red and Green, which were released in 2004. Based upon FireRed and LeafGreen being nearly exactly the same as Red and Green as far as storyline goes, the assumed storyline of the remakes would thus follow Gold and Silver very closely, with the player starting in New Bark Town and traveling across Johto, and later Kanto. Though disputed for a long time, these suspicions were finally confirmed with the announcement of the remakes themselves in May of 2009.
Shortly after the release of the remakes of Generation I's paired versions in 2004, some fans began to expect a second pair of remakes, this time of the Generation II games, as, like Red, Green, Blue, and Yellow, Pokémon Gold, Silver, and Crystal were incompatible with the then-current Generation III. Unsubstantiated rumors and theories began to spread around forums online, and some fans went as far as to make hacks of Generation III games to "remake" Gold and Silver. On April Fool's day, 2004, Pkmn.Net showed faked screenshots depicting the Daycare on Route 34 in a remake, claiming that the screenshots came from CoroCoro.
Those who wished for remakes also pointed to allusions to Gold and Silver in Generation III's five games. For instance, Giovanni is revealed by a Rocket Scientist in the Sevii Islands to have a red-haired child, widely speculated to be the Generation II rival, Silver, while Bill's information in the Fame Checker mentions his hometown of Goldenrod City.
Notably, however, FireRed and LeafGreen were used to provide many Generation II Pokémon to Generation III games, as most were missing from the Hoenn Dex (and thus not available without trading in Pokémon Ruby and Sapphire), while the only Generation III Pokémon available were Azurill and Wynaut, themselves breedable from Generation II's Marill and Wobbuffet. An NPC even gives away a Togepi egg, as Elm does in Generation II, and aside from Pokémon Colosseum and Pokémon XD, this is the only way to get many of these species.
Despite the theories of the many, the time of the Game Boy Advance passed, and Generation III ended without a remake. With Generation IV and the Nintendo DS, however, those who were not discouraged transferred their theories, speculating for a remake that would be released some time after Pokémon Diamond and Pearl. Like in Generation III, several mentions are made during the course of Sinnoh's games, some more obvious than others, of events and locations in Generation II. Examples of this include a news report of the red Gyarados, Jasmine's appearance in Sunyshore City, an explicit mention of Professor Elm and his report about eggs, Cynthia's SecretPotion (which mentions Cianwood City in its bag description, as it did in Generation II), Bebe's friend from Johto who has many Eevee (possibly Bill considering their similar line of work), and the man in the Hotel Grand Lake who mentions Ho-Oh's roost of the Tin Tower and Lugia's home of the Whirl Islands and the fact that Clefairy dance on Monday night in Mt. Moon.
The only concrete clue, however, was hidden, in-game data for the locations displayed on a Pokémon's status screen. While a Pokémon caught in Sinnoh will display the area it was caught in, and Pokémon sent from Generation III via Pal Park display "Hoenn" or "Kanto", depending on the game in which they were originally caught, the data also includes "Johto", at the time inaccessible through any means.
Alongside the more concrete references listed above, there are a few "hints" theorists have pointed out that are far more speculative. Some examples follow:
When FireRed and LeafGreen were released, Ken Sugimori created new stock art for all of the Generation I Pokémon and a select few Generation II Pokémon (such as Togepi), making those that did not receive new art (such as Scizor) look out of place next to their evolutions and pre-evolutions of other generations. A new remake would, like FireRed and LeafGreen, encourage this art to be updated. As shown in the Pokémon Sunday announcement, the stock art for the remaining Generation II Pokémon will be updated.
It is also becoming completely impossible to play the Generation II Pokémon games for any length of time. Unlike later games, which rely on flash memory for their save files, the save files on both Generation I and II games are maintained only by an internal battery on the cartridge itself. Normally, these batteries can last for decades, however, the constantly-running clock of Generation II drains these batteries more quickly than in Generation I games, causing save files to be lost and unable to be recovered. As Pokémon Diamond, Pearl, and Platinum's clock runs off of the Nintendo DS's system clock, this issue is eliminated completely.
With the addition of the legendary birds and legendary golems in Platinum, as well, it seems that the games are moving away from reliance on Generation III to complete the Pokédex; this means that other Pokémon missing from Platinum, and not in Diamond or Pearl, such as the legendary beasts or Ho-Oh and Lugia, could theoretically be supplied by remakes. There is also no legitimate way of getting Celebi outside of Japan without trading with someone from Japan; being a Generation II Pokémon, it could be given out in an event much as Mew was after the release of FireRed and LeafGreen.
The latter theory is strengthened by the release of the Nintendo DSi, which lacks a GBA slot completely, cutting players off from their Generation III games. Not only is Pal Park unusable, but dual-slot mode also, leading to players either having to own both an older model of DS as well as a DSi, or necessitating trades with players who have these older models. When this was pointed out by a reader of Nintendo Power, the editors suggested that this issue would keep sales of older DS models strong.
Many events in the anime can and have been interpreted as references as well. All three of the current main characters of the Diamond & Pearl series have or have had at least one Generation II Pokémon on their team during the series up to this point, Ash with an Aipom he later traded to Dawn and Gligar, Brock with Sudowoodo, and Dawn with Aipom and Swinub. While these Pokémon were at one point or another during the series also a Generation IV Pokémon (all being related by evolution to one), Aipom had 60 episodes during the series before evolving, while Brock's Bonsly evolved soon after the series began, and has been in its Generation II stage ever since. Team Rocket as well has Generation II Pokémon in Wobbuffet (which has been around since Ash was in Johto) and Yanma (which evolved into Yanmega shortly after its capture).
The main characters are not the only ones with these supposed references, as Zoey, Dawn's rival, owns a Misdreavus and Paul, Ash's rival, has a Weavile, an Ursaring, a Gliscor, an Electabuzz, and a Magmar, all of which are evolved from Generation II Pokémon. His design is also very similar to Silver's, with the stock art even being in the same pose, and fans theorize that he is related to Pyramid King Brandon, the final Kanto Frontier Brain much as Silver is possibly related to Giovanni, the final Kanto Gym Leader. Butch and Cassidy, likewise, are revealed to have a Shuckle and Granbull, respectively, in their first (and so far, only) Sinnoh appearance.
There have also been several cameos of the character Marina from The Legend of Thunder, who is based on Kris, the female player character from Pokémon Crystal. As all games since Crystal have had the choice of a male or female player character, she would presumably be the female choice in remakes, even though only Gold was in the original pair.
The badges that Reggie, Paul's older brother, won have also appeared in the anime, and were scrolled across before his match with Ash: the camera goes leftward, focusing on the badges of Kanto, then Hoenn, then Sinnoh, and finally to Johto. One can assume that, since it does not go by generation (putting Johto between Kanto and Hoenn), it goes according to recent games: Pokémon FireRed and LeafGreen, Pokémon Emerald, Pokémon Diamond, Pearl, and Platinum, and then next, on to a Johto-based game.
Gary Oak, Ash's old rival and anime counterpart of Blue, the Generation II Leader of Viridian Gym, has also made several appearances in the anime, possibly as a reference to his counterpart's profession. In addition, the Viridian Gym, when it was last seen in The Scheme Team, is looking for a new leader after Giovanni apparently abandoned it, which could provide an opportunity for Gary to take the spot his counterpart has.
Several other hints in episode plots have also been seen, such as that of Ya See We Want an Evolution, where Jessie, James, and Meowth attempt to evolve a Magikarp into Gyarados using a machine; a similar incident resulted in the red Gyarados in the Generation II games.
It should be noted that as with the games, there is no need for the anime to go back to Johto unless a similar situation to the Battle Frontier saga occurs in Johto as Ash has already acquired all of the badges in the region; this now looks more likely due to the announcement of a new Battle Frontier, that is likely to be featured in the anime as the other one was. Each new facility has been shown to have a Frontier Brain, which would give Ash the incentive to challenge them. Dawn has expressed interest in the Johto contests as well, mostly during the Wallace Cup.
One of the featured Pokémon of the twelfth Pokémon movie appears to be a unique Pichu, which was an iconic Pokémon during the release of Gold and Silver. As revealed by another poster with Heatran on it, the three Johto starters will also appear, seemingly at random.
Ho-Oh and Raikou remain the only Generation II legendary Pokémon who have not received their own film role, unless one considers the one-hour Raikou special a movie and Mew's transformation into Ho-Oh during Lucario and the Mystery of Mew as film roles. Ho-Oh has appeared in the show repeatedly, but if the anime were to return to Johto, these two may finally receive a proper film appearance.
Background music from the Generation II games has also been used extensively recently, with Ash's first meeting with Fantina having the theme of Routes 26 and 27 playing, and his training battle against Dawn in Chim-Charred! using the battle theme of Red, his game counterpart. The final anime episode that aired before the remakes were announced, DP126, also featured the background music of Route 29, Johto's first route, as well as a Cyndaquil in Candice's school.
Critics of the remake theory held that what theorists point to as evidence in Diamond and Pearl often consisted of cross-series references, and that these result from the fact that Generation IV takes place around the same time as Generation II. In addition, they pointed out that all game-related "proofs" were simply fan speculation, save the in-game "Johto" location text. When confronted with this, critics reply that the Pokémon developers have a history of leaving unfinished and unutilized data and code in the Pokémon games. There is also "Sinnoh" location text, despite every location of Sinnoh having its own data. The S.S. Anne ticket within the same games proves this point.
References to Johto in the anime are generally considered to be irrelevant, as the Pokémon games rarely match up with other forms of media; no game since Pokémon Yellow has been significantly influenced by the anime. Beyond that, there had been almost as many references to Hoenn as to Johto (aside from Pokémon themselves).
Many believe that FireRed and LeafGreen were initially released to make available Kanto Pokémon not found in Ruby and Sapphire or Colosseum; with all of the Generation III and IV games released so far, every Pokémon is available in some way. Theorists defended against this statement by pointing out that filling the Pokédex in this manner requires a player to have access to all the games for both Game Boy Advance and Nintendo GameCube, in addition to a cable to link the two and a GBA itself. Also, they pointed out that Pokémon XD made available all of the Kanto Pokémon that weren't in the Hoenn Dex, even though FireRed and LeafGreen had already made these Pokémon obtainable.
Many names were theorized by fans until the announcement of HeartGold and SoulSilver; however, a majority were in the format "XGold" and "YSilver", following the pattern of FireRed and LeafGreen. Because of the multitude of names theorized, and because there were not any "official" names yet, the Bulbapedia user TTEchidna coined the term "GSDS", as they would be remakes likely on the Nintendo DS.