Body mods for shark kin and star kin?
Gill tattoos! I think these are the coolest thing ever and I need to see more or them. You could also do them on the neck for a more mermaid vibe.
Permanent fangs! You could get two or four done permanently for a more subtle look and have a fake set for the whole mouth. Or you could go all out and get them all done, but it would be harder to find a (good) orthodontist who would do them.
A tattoo of a dorsal fin would be cool, but probably impossible. You could potentially do one of a scar, like it had been cut off
There are so many options for star tattoos, and you can do them in more than one placement. If there's a constellation you resonate with, you can also weave it in there.
White ink would also look epic but it doesn't last very long.
You can also get them done in brown ink to look like freckles, like you were just born with them naturally.
For hair, you could do any iridescent colors
Glow in the dark would also be really cool
You can also form stars with your part.
If you have the texture, you can also sculpt it into stars!
And finally, celestial makeup like your eyes are the center of a supernova.
I hope you enjoy, and please come again!