Here’s the official artist’s description.

She has the magic of Celestia, Luna, and herself inside of her. As a result, she’s taller than Celestia and is more powerful than Celestia and Luna combined. Her ruling Equestria I think is basically like a second ascension, so the 4 wings and 2 horns make her extra special and powerful. The Rainbow Power appearance is her friendship side. She can fly much faster and her weather manipulation abilities are much more powerful than the average Alicorn, her Earth Pony strength is stronger, and the 2 horns allow her to cast spells faster than the average Unicorn or Alicorn. She can even use her horns to combine spells in ways that Unicorns or Alicorns can’t. However, the extra wing power and horn drain her energy and magic faster, but she already has a LOT of energy and magic reserves already, so the increased energy drain shouldn’t be much of an issue.

One thing she can do is absorb the friendship power of her subjects to unleash a rainbow blast like what the Elements of Harmony shoot at will. Her power and the length of her mane and tail are tied to Equestria’s friendship power. If her subjects have lots of friends and everyone’s friendship bonds are strong, she’s more powerful and her mane and tail are longer. If her subjects don’t have a lot of friends and everyone’s friendship bonds are weak, she’s less powerful and her mane and tail are shorter and may not glow or even move anymore. She can also draw upon her subjects’ friendship power to heal wounds and regenerate lost limbs really fast. The only way a villain is going to be able to defeat her is to first break her subjects’ friendships. Unfortunately for the villains, since she’ll be promoting and sharing friendship throughout her kingdom, the villains are going to have a much harder time doing that than in Season 9.

Her mane and tail are also extra special. Her mane and tail are overflowing with friendship magic. Anyone who touches her mane or tail will feel the best they’ve ever had and all of their stress and sorrow will be washed away. If someone not pure of heart touches her mane or tail, they’ll be afflicted with a nasty burn. Her mane and tail can also act as a super shield and absorb and nullify any spell that touches them, including dark magic. She can exploit this ability to shield her entire body with her super long mane and tail. However, she can’t move or attack while shielding her entire body, but she can move her mane or tail in front of an attack to block. Obviously, her mane and tail can only do these things if her subjects’ friendships are strong.