Corrupting everything because corruption is what the villain is the literal manifestation of. I'd like the main villain to be based around negativity corrupting the forces of Light and Darkness, with corrupted light as it's main tool, the minions basically light based versions of the Heartless, and the other villains mostly along the Well Intentioned Extremist and Knight Templar bent. We've already faced a lot of Dark Is Evil villains with the Xehanort saga so I think it would be appropriate for us to shift to Light Is Not Good, and give the Light it's chance to shine... and burn everything to ashes. The end idea is that neither Light or Dark are evil, as the games have hinted at, but that negative feelings corrupt either, eventually resulting in it becoming a force of their own, ergo beings like The Heartless come into being.