Digimon Musou by Tecmo-Koei
Plot focuses on the kids from Adventure, 02, and Tamers (the three most popular series) trapped in a world where they must work together to fight their greatest foes again, battling the likes of Devimon, Arukenimon and Mummymon, and the Devas plus their armies of virus-infected Digimon in order to escape back to their own realities. Mostly the scenario mode would you have you relive events from the shows, but with different Digimon, and such---like what a lot of Musou titles do. You'd have characters from the other series as well appearing as playable, but with less focus in the story mode and less characters per series.
Gameplay is simple Musou, but instead of string-combos like usual it'd go for a simpler set-up with each Digimon having a basic melee combo, as well as two special attacks, a guard, and a jump. Throw in a basic RPS-system based on the Virus/Data/Vaccine relationship and you're golden. The Appmon would make an appearance as collectible items, which can they equip to the playable Digimon and grant them useful abilities. Otherwise it's typical Musou, go around a map completing objectives while plowing through enemies.
The biggest gimmick would of course be Digivolution, with the majority of the cast starting as one Digimon but by filling up a Digivolution bar could transform into another form for a limited time. Here's my idea for the roster, mainly focused on Adventure, 02, and Tamers...
Adventure Digimon have a linear transformation, beginning as Champion and changing to Mega---they then could stay in that form till the bar runs out, or unleash a screen-clearing move to revert back.
*Tai and Greymon -> Wargreymon
*Matt and Garurumon -> Metalgarurumon
*Sora and Birdramon -> Hououmon
*Izzy and Tentomon -> HerculesKabuterimon
*Mimi and Togemon -> Rosemon
*Joe and Ikkakumon -> Vikemon
*T.K. and Angemon -> Seraphimon
*Hikari and Gatomon -> Ofanimon
*Tai and Matt + Omnimon: A secret character, he's very powerful. When his bar fills he can unleash a screen-clearing move.
Adventure 02 Digimon can switch between their Armor forms whenever they want. Though they aren't as strong as the Mega Adventure Digimon, they make up for it with more versatility. When their bar fills they DNA Digivolve and perform a screen-clearing move.
*Davis and Flamedramon <-> Raidramon (DNA Paildramon)
*Yolei and Halsemon <-> Shurimon (DNA Silphymon)
*Cody and Digmon <-> Submarimon (DNA Shakkokumon)
*Davis and Ken + Imperialdramon Dragon Mode <-> Fighter Mode: A secret character, he can freely switch between modes at his choosing. When his bar fills he turns into Imperialdramon Paladin Mode and unleashes a screen-clearing move.
Tamer Digimon begin as weaker Rookie Digimon and can only evolve into Champion Digimon when their bar fills, but then they stay as such. Each fight you can also select four Digimon Cards (each Tamer Digimon has their own selection which increases as you play) as their load-out granting them unique powers. If you fill the bar a second time they can then unleash a very powerful screen-clearing move as a Mega, and then revert back to Rookie.
*Takato and Guilmon -> Growlmon (Gallantmon)
*Henry and Terriermon -> Gargomon (MegaGargomon)
*Rika and Renamon -> Kyuubimon (Sakuyamon)
*Beelzemon -> Beelzemon Blast Mode: A secret character, he can't use cards but instead can either run on foot or ride on his motorcycle Behemoth, his choice, which alters his moves. When his bar fills up he turns into Blast Mode, making him even stronger and granting him flight, and will revert back when the bar empties or he uses a screen-clearing move.
Frontier Digimon begin as their Human Spirits and can transform into their Beast Spirits at any time, but will automatically revert back if their bar drops to zero as they burn through the energy quickly. Beast Spirits are extremely fast and powerful, but quickly eat up energy and will lose health if you let their bar run out, so you'll want to balance it. If you get the bar up all the way you can unleash a screen clearing attack as their Transcendent Forms.
*Takuya as Agunimon <-> BurningGreymon (Trans. Kaiser Greymon)
*Koji as Lobomon <-> KendoGarurumon (Trans. Magna Garurumon)
Savers Digimon begin as Mega level, and have a bar known as the Burst Bar which they can use to unleash powered-up special moves as their Burst Forms and special guards, or save up for a screen-clearing attack.
*Marcus and Shine Greymon (Shine Greymon Burst Mode)
*Thomas and Mirage Gaogamon (Mirage Gaogamon Burst Mode)
Xros Wars Digimon begin as "Rookie" Digimon, and have more traditional Musou-style string attacks with each finisher causing them to transform into one of their composite forms. When their bar fills up they turn into their strongest combination and unleash a screen-clearing move.
* Mikey and Shoutmon (Shoutmon X7)
