Doesn't it look like Riku has a light aura? If people are seeing this the way someone else did on another thread, thinking that the trailer sets Sora and Riku up as opposites, then drop could refer to Sora's "fall" to darkness and Riku's "fall" to light. If the drop gauge refers to how powerful or the ability to use powers outside of their original repitoire, then Riku could be gaining light powers and Sora could gain dark powers.