I wouldn't mind seeing something along the lines of DBZ Boudokai, a fighting game where you unlock various cards through fights, and only certain people can use certain cards.
You'd start out with DK, WK, Torque, Incizor, and maybe Camo, and unlock them by going through story mode and beating the others.
I'm not sure if you could make Xaviax playable this way. If he were, he wouldn't have any cards at all.
The card setup would be very similar to the capsule setup in the Boudokai series, with different cards doing different things.
You have your action cards, such as sword vent, or strike vent, that are set at the beginning of the game, and are used by doing a certain button press. This includes the final vents as well. And similar to boudokai, the final attack may not necessarily be the end, but just a really powerful attack the the enemy may be able to block or at the very least take half damage from.
Then you have your defensive cards that are set and used with a single button press. These include shield vent, which would block so many normal attacks before breaking, or trick vent which would possibly stop the attack, and also leave the enemy open for an attack. Cards like nasty vent would fall in the catagory as well.
And then you'd have your special cards, such as Such as Wrath's Time Vent, which are extremely powerful and do various things. DK and WK's survive forms would be a part of this, similar to say the patora earrings or the transformations in Boudokai, a Survive form card would take up all the AP for the cards of that character, but would give you an incredibly powerful character with his own list of different abilities to use.
Of course you wouldn't be able to load all the cards onto one character and go at it. Each card is specific to each character, so you would have to unlock thae card for that character to use it. also there would possibly be point system similarly to the little used AP point system in the show, where each card cost a certain amount of that characters AP.
As for characters, all of Dragon Knight characters would be there, once you unlocked them though. Xaviax might be a special unlockable character who uses has cards based on the different enemies you fought or something like that.
And what the hell, why not throw decade into the mix as well, make him a very special "Kamen Rider from a Foreign World" type character just for the fans like us. He'd have his own cards, and possibly the ability to turn into other Ryuki characters as well.