How about new weather moves? I'd like to see one day is Darkness as a weather. Yep, the kind you've encountered in caves since Gen. I and (never bothered to) banish with Flash. Maybe a move called Blackout?
Don't know too much about its battle effects, though. Knowing GameFreak, they would probably give it accuracy-reducing effects. This would suck for the metagame, but that isn't very important in the big picture. We'd send it the way of DrizzleSwim, Double Team and Brightpowder. Us banning it wouldn't prevent GameFreak from putting it in their games.
Darkness could just as well be a boon to Ghost- and Dark-Pokémon too/instead. It could boost any single of their stats except for Speed and Accuracy, and I'd be content (NOTE: Not all of them at the same time. I just say that it could work just as well for either raising their evasion, their offensive stats, or their defensive stats).
Most Dark moves have pretty low base power, so perhaps it could work well as an offensive boost, similar to Water moves in Rain and Fire in Sun.
It could even work as a bonus for "sneaky" playstyles. For instance, doubling passive damage or increase the chance of side effects happening. After all, everything gets worse in the dark.