Maybe the Maleficents fuse into a Hydra DragonOK, I have more thoughts about the Warriors game concept:
-Guardians of Light focused game, sort of the spiritual successor to Days being Org XIII focused.
-Timeline wise is set simultaneously with the Sora/RIku/Yozora game. Essentially this is the "meanwhile" game.
-Data-Sora & Data-Riku are involved later in the story so they substitute for the real deals.
-Maleficent actually is a major threat & driving force of the plot, especially when there isn't too many other bad guys really doing anything atm. She spent the year-long time skip building up the power needed to pull off a larger scale version of her put-the-kingdom-to-sleep spell from Sleeping Beauty. This time putting the entire damn Realm of Light to sleep, trapping every world in the Realm of Sleep in the process.
-Heartless, nobodies, & especially dream eaters are the common enemy forces. World specific threats like card soldiers, goons, or Black Guard can also appear. Spirit DE serve as the "good" army that fights along wise the GoL.
-Maleficent's sleep magic extends to giving her control over the nightmare DE in addition to heartless. Luxu retains control over nobodies. How is it he can do that while Lea/Isa/Roxas can't?.............Anyway the other Foretellers don't have minions of their own but those that agree to side with Luxu get nobody support.
-Bosses include returning heartless/DE/Twilight Thorn plus dream/phantom versions of past Disney villains. Most battlefields are based on old Disney worlds without really any new ones.
-Over time the Datascape is involved as Maleficent & Foretellers are both looking for it, hence the arrival of Data-Sora & Riku. Shit happens & the DE of both sides get turned back into keyblade warriors. Maleficent is defeated around this point. Her boss fight has her teaming up with dream & data versions of herself, so the player has to fight 3 witches/dragons. The worlds are returned to the RoL.
-Foretellers take over the plot now as the Keyblade War is reignited. GoL are forced to choose sides. Maybe they could even be factures & infighting within the GoL as they become split over the war. Though D-Sora, Kairi, & Mickey end up siding with the returning Ava in bringing peace.
-Lauriam and Elrena probably get involved somewhere along the way. Maybe a DLC scenario since there's a lot going on as is.
-Luxu is final boss as he is fully taken over by a darkness. Uses both keyblade and arrowguns.
-Post credits scene has remaining darknesses in Datascape escaping into RoL.
TL,DR: Wherever Nomura was going with the Foretellers saga is done in musou format instead of a mainline game as to better convey the scale of the Keyblade War.