Darkness does not mean evil, sample : Riku, he fight for the good, and use Darkness power.
Light does not mean good : sample : Zexion, he used a wall of light to fool Riku, and the heartless in End of the World : they use Light.
The things is that, we mainly showed to us Light used for Good, and Darkness for evil, nothing proves it is impossible to see an ennemy based on Light.
We are always in Light or in between world. Light is Water and Darkness is Fire. Darkness in Light World spread and the water evaporate, so, the light, Light in Darkness world is the water and Fire disapears, so, Darkness. Is on or the other evil??
I think we will see Light ennemy more likely in KH3 though, but the idea is very likely.