[PantherPrincess]

I think it'd be neat if light and dark got their respective enchants through badges and/or mages enchanting skills. Their weaknesses would obviously be each other and it pretty much works the same way as every other element does.

Uranus as a badge would grant +1 Faith +1 Will and as an enchant +11 Light Attack +4 Faith/Will
Pluto on the other hand would receive +1 Res +1 Will and as an enchant +11 Dark Attack +4 Res/Will

Skill effects when enchanted are pretty obvious too. Uranus would create sanctuary tiles (or light shafts) when attacking or using skills such as sidecut.
Pluto would create dark water tiles when attacking or using skills.

[Neus]

I recommend searching for this suggestion. It's come up a lot and the reasons I haven't already done it are still very relevant.