Um, maybe you could do good deeds (e.g. freeing people from the heartless) to get your meter down.
Or, maybe, a point system like in Fable.
OFF TOPIC : lol, I'm making this post on my Wii
Another thought: If Sora remains inactive whilst at full darkness, a cooldown meter will appear, when it reaches the bottom, Sora's darkness meter goes down.
Yays, I have ideas :D XD
Ok, I have an I idea based on some of the clues we have received from the KH2 and KH2 FM+ secret endings that there will be two styles of gameplay:
1- The Ordinary KH1/2 battle system with new twists ( Such as, possibly, a "darkness meter" of some kind based on the tinges of corruption in Sora which have become to surface [ Craving to be king in pride lands etc...] )
2- A Large scale "Rome Total War" Style gameplay where important characters are generals and they control less important characters ( My idea for this is based upon the phrase " Keyblade War" which appears in the KH2 Secret Trailer with Ven, Aqua and Terra)
I believe this idea would suit players if they could choose between which one they want to do.