In Digimon World Re:Digitize, the game allows you to view a summary of previous game events and separates them into chapters. In Decode, this figure has increased to twice the original number. The events chosen were those that the director and producer felt would appeal to Digimon fans as well as those they felt balanced the main scenario out. They stated that one of the challenges that they faced was incorporating the popular X-Antibody and Demon Lord storylines into the game while staying true to the previous material that already exists.
These arcs were introduced into the game such that the game transitioned seamlessly from one arc to the next. For example, the end of the Vitium arc opens directly into the Demon Lords arc. As such, changes were made to the main scenario (the entirety of the PSP game) starting from the prologue, and enormous changes were made to the opening to give a different “atmosphere” to the game.
Creator preference was also involved. The producer wanted to balance out the original arc, which was about the battle against the Vitium, a non-Digimon entity, so he and the director chose to write about a battle between Digimon. This resulted in the Demon Lords arc.
Another change they wanted to make even in the PSP game (but couldn’t fit it in) was the Infinity Mountain dungeon, which appears in the X-Antibody arc. In general, there are no proper “dungeons” in Digimon World Re:Digitize—only fields. However, in Decode, Infinity Mountain was recreated to have a completely different playing style and remade into a dungeon with a different playing style to make it more fun for players. Instead of a simple 3×3 randomly generated map, proper maps were created, complete with traps.
The game’s producer was especially enthusiastic in the interviews he participated in, about including the good ol’ pitfall trap, although he mentions there are some traps added purely for humor as well. (Digimon Adventure episode 24 comes to mind for some reason.)
Infinity Mountain is also 100 levels. The dungeon was created so that, while it would be challenging, it wouldn’t be impossible to clear for most people so long as they put in some effort. Save points also appear periodically throughout the levels.
To make the going easier, you’re given the option to retrieve Digimon whose data you’ve stored in the Digitorage before they died. This means that you can use favorite Digimon you’ve previously raised (but have since died) in Infinity Mountain to accompany you on your journey.
Because of the different playing style, the creators also took the opportunity to introduce another feature that doesn’t appear outside of Infinity Dungeon. In Digimon World, Digimon always appear around the same size after a certain point because of game (or screen) limitations, despite some Digimon, like MugenDramon, being enormous. However, in Infinity Mountain, the Digimon were made to appear as large as they did in the anime. (It is unclear if this refers only to enemy Digimon or also partner Digimon.)
Also, the creators felt that the effects of the irregularities in the Digital World in the Real World weren’t clearly shown. Because the events obviously aren’t in the Digital World, there was no way they could write these in. As such, they decided to use a different perspective. To explain the events in the Real World, they had the new character, Rina Shinomiya, comment on events in the real world and explain what’s going on starting from the start of the game.
Unlike the protagonist, who blazes through the Digital World, Rina was more “the second protagonist who watches over the events from the human world.” This is why, as per tradition, she wears goggles like the majority of the previous franchise protagonists.
In addition to extra events, the number of Digimon was also greatly increased to 162 Digimon that you can raise. This explosion of new Digimon numbers was partially decided by a vote by fans. One of the most popular suggestions were “Digimon that had appeared in the anime,” and V-mon was a special favorite. Another popular suggestion was “Digimon that had appeared in the first Digimon World game,” and this led to the inclusion of Tyrannomon, among others.
Tyrannomon, especially, received favorable treatment. He’s been a fan favorite ever since the liquid crystal times, even appearing on some of the covers, but he hasn’t been featured much since. This time, the creators finally created a set final form to Tyrannomon while discussing with Digimon designer Kenji Watanabe, creator of the original Digimon such as Devimon, and this resulted in RustTyrannomon.
Another Digimon to receive a new form in Decode is Ogremon, who was made very popular from the anime. However, because he was so popular in Adult form, no Ultimate form was ever created for him before this. His new final form is called Titamon.
Adding so many new Digimon was one of the harder aspects of creating the Digimon World game, note both the director and producer. According to the producer, in a usual RPG, if you have approximately 50 enemy monsters, you can make a working RPG. If you use recolors, you can have further variation. However, with Digimon, every single Digimon has their own personality and shape, so their design, their poses, and their actions would all be different. Even among the Human Digimon, their body shapes are different, so every last one would have to be created separately.
As such, dividing the time and budget was difficult. The producer stated that he wanted to increase the richness of the Digimon’s actions, such as creating unique eating motions, increasing the number of battle attack motions, etc., but this would decrease the number of Digimon that could appear, so he decided against it.
Source: Famitsu and Dengeki Online interviews.