The idea is that Nami is star constellation. So she a celestial (or SEAlestial heehehe) being. She would maintain her mermaid body shape and probably have a different head (dorsal?) fin. The coloring would be a deep purple/black mostly, with nice white stars to help out fill in the shape and pronounce her facial features. Her staff would feature a spinning moon on top of it.
Her Q would be like a black hole.
The w would be either a comet with trail or be the colors of the milkyway.
The e would put asteroids around the champion.
The R would be the colors and fluidity of a solar flare (not to be confused with leona.)
If any artists want to take a shot at this;even just a recoloring of the current nami as a reference, please feel free to do so and post it here. I'll update the post with it and be sure to give you the credit. If you like the idea upvote it so more people see!.