Any entity that represents/controls one domian has an extreme opposite. This means Nightmare Moon isn't the only possible baddy.
However she is the first we have seen. Not only does she desire eternal night (which not only slows Gaia's rotation but places the moon in permanent solar eclipse position) she desires obedience and worship. Once the whole planet is engulfed in darkenss, her power would increase tengold and in a flash of fury and the need to be the ultimate being, she would begin the slow and painful transformation of the entire planet's population into some form of bat pony or nocturnal creature. They would not only worship her and the night in which they lived, but they would rely on her- too afraid of and sensitive to the sun that reversing her acts would be deemed impossible (if not really really really hard to do). Then its just her reigning over the whole world with an iron fist. A constant militant state of nocturnal beasts.
Luna's sister ion the otherhand, when out of control is known as Daybreaker and perhaps is more ruthless than her sibling. Daybreaker cares not for power over others. She cares about fire. Her glorious and beautiful star is her muse and she longs to make the pitiful rock she is stuck on to be as similar as possible (cuz she can't live on the sun it makes her grumpy). For starters the water has to go. Oceans are evaporated and leave enormous salt valleys and trenches dividing continents and killing all aquatic species. Next, anything that sucks up excess oxygen that could be fueling her fire must go. Trees and vegitation scorched down and any animal caught in the way as well. Magma brought up from the core to coat the soft, pansy soil in a layer of burning crust. It could be possible to survive Daybreaker's world, hypothetically, but it would suck. One would have to live very very far beneath the surface where there might be water still present and have a way to produce oxygen for themself. it would be very hard to do and if Daybreaker ever found you she'd immediately wreck your shit.
Now if Cadence lost her marbles she'd go all grey and stoic. All the love and emotion would nearly vanish from the world and procreation would come to an end. Basically the only things that would continue to live would be immortals and anything that doesn't have emotions. Everything else would slip inot complete apathy unitl they died. I'd imagine she would have come close to this when Shining Armor died.
It is extrememly unlikely since Discord exists on the mortal plane, but there could be the chance that Twilight turns and creates the perfect, harmonious world. There wouldn't be much to it, just a drone like existence where evrything is micromanaged to be perfect and everyone is a mindless drone that does everything they are supposed to. Eat, drink, move, reproduce, die- repeat.
Each one of these lovely ladies would surely be willing to step up and stop the other(s) is things got out of hand- as Celestia already has for Nightmare Moon. But that does not mean that they aren't possible outsomes. In such an instance as these 100% of all their magical power would be put forth towards their goal. Their maximum potential would be release- which is A LOT. So much so that it would probably take more than one to oust another. Two alicorns may be the minimum if Daybreaker was to make an attempt.
So yeas this is my headcanon. Fire away.
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