Ultimate Skin Concepts?

What are some good ideas for ultimate skins? This isn't about who deserves an ultimate skin, but rather, what are some ideas that would warrant an ultimate skin.

Ultimate skins need to have multiple models that can be used in the same game, either chosen by the player or representing progression throughout the game.

For example:

Stellar Incarnate Aurelion Sol He begins the game with a very nebulous appearance, with his body parts very separate and "floaty", mostly composed of colorful mist. His abilities would also have a very nebula-like look.

As he leveled up, he would get a choice to either become a normal star or a supermassive star.

A normal star would give him a yellow coloration on his body and abilities, as well as the stars that circle him. Then, he would move into a red giant phase, turning red and beefier. Late in the game, he would then go into a planetary nebula/white dwarf phase, with a very regal and elegant appearance.

If he instead chose the supermassive form, he would be blue and bright. Later, he would become a supermassive red giant, and near the end of the game would turn into a black hole, with a very dark and evil-looking appearance.