One main theme could be isolation. The title itself implies a sense of being alone against the rest of the world.
One main theme could be the struggle against a cold and unfeeling society. The protagonist might be constantly facing the apathy of the world around them.
Well, in Against the Gods novel, the themes include growth. The main character grows from a relatively weak state to a very powerful one over time. Also, loyalty is a theme as he has friends and allies who stay with him through thick and thin. And revenge can be considered a theme too as he has his own scores to settle with certain enemies.
There could be a secret organization that has managed to turn the world against Naruto. This organization could be made up of rogue ninjas or people with a vendetta against the Uzumaki clan. They might spread false rumors or manipulate events to make Naruto look like a villain. Another possibility is that some of the old - school shinobi leaders who are afraid of change and Naruto represents a big change in the ninja world, so they oppose him.
Self - discovery. Maybe through going against the rules, Buttercup discovers something new about herself, like hidden powers or different aspects of her personality.
One possible theme could be humor. Since both MLP and Cards Against Humanity have elements of humor, the fanfic might play on the funny and unexpected combinations of MLP characters and Cards Against Humanity - style statements. For example, making jokes about the different pony tribes in a Cards Against Humanity - like way.
Power and growth. The protagonist constantly pursues greater power and goes through various challenges to grow stronger.
One of the main themes could be the struggle against various foes. It might show the protagonist's fight against different types of enemies, be they physical or ideological.
One of the main themes could be struggle. The characters might be facing various difficulties and fighting against odds. For example, they could be dealing with personal hardships or societal challenges.
The main theme could be about an individual's struggle and determination to overcome insurmountable odds and prove their worth against a hostile world.
The main plot could revolve around the princess's journey to assert herself in a world that is against her. It might include her battles against enemies who want to dethrone her or those who oppose her progressive ideas. For example, she could be trying to bring equality to her people while the traditionalists are against her. She may also encounter personal struggles like finding true love in the midst of all the chaos and power struggles.