Looking at this I've got two ideas about the story and gameplay.



Story



If I had to make a guess of the story for such a game, I'd say it'd be a altered version of Volume's One and Two as one whole story. Beginning with the robbery of the Dust till Dawn and ending with the Grimm assault on Vale.



The plot would follow Team RWBY as they try to figure out and counter the actions of one Roman Torchwick a notorious criminal mastermind whom has allied himself with the infamous White Fang, a Faunus Terrorist/Civil Rights Group. Throughout the game, you'd discover various files, scrolls and other things that point to a bigger picture and you begin to wonder, just WHAT is going on? Why has a Criminal Mastermind, whom until now has just been focused on controlling Vale's underworld allied himself with a Terrorist Organisation and is trying to get the Creatures of Grimm into the city.



By the end of the game Torchwick is captured and Vale is at - relative - peace again, but both you and the girls are left with more questions than answers, but they resolve that they'll find out eventually.



Then, in the Post-Credits Secret Ending, Cinder, Emerald, Mercury and Adam meet up in the Warehouse from the end of "The Best Day Ever" to discuss their situation. Then Cinder contacts Roman via a hidden transmitter, informing him that: "Everything is going according to plan". Cue Title Screen!



Gameplay



As for the game play, I'd say that each of the four would have an individual fighting style and role in the gameplay based on their weapons. As for controlling them, I'd say that you'd be able to switch between the four at the push of a button, controlling them until one of the buttons is pressed again.



On certain missions, such as the flashback trailer missions however or the ending train mission, you'd control one of the four individually in combat, using their individual skills to complete the level.



As for the team attacks: Ice Flower, Checkmate, Ladybug, Freezer Burn and Bumble Bee - including others - would be activated by selecting the teammate in question and confirming your team attack request.

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To be fair, I'm getting a very Kingdom Hearts feeling from this idea.