Here's the plotline that I would like to see:
The evil forces have lied dormant collecting the power so that they can transport themselves through time. Sora, Riku and the King are too strong for them to take on. The three keyblade wielder's shown in the end of the credits after the perfect completion then decide that to achieve their goals they must go back in time. To a time where there were no heartless and no nobodies. Then, they can take control without the fear of the keyblade wielders. Changes are made to the world's over time as they go from peaceful to anarchy, and as the game progresses they find the indicators that suggest that things are changed by the evil doer's of the present and are committed in the past. From there, they must find the door to the past in each world and right the wrongs committed. In the end, it comes down to a full-scale war. The three keyblade wielders of light against the three keyblade wielders of dark. That's just a brief version of what's on my mind that they need to go with. They need war, drama, love, despair, distraught, and the true power of the keyblade unleashed: The key to their hearts. After all, they made it abundantly clear that there's where all the power is coming from.