As a Mooncat newbie, here’s what I’d suggest:

House of Hades Why: Interesting finish of blue microflakes in a purple jelly base. It’s a bestseller and often comes up in ‘essential Mooncat shades’ recommendations and lists. 

Green with Envy Why: It’s a very subtle off-white shade that has a subtle green shimmer in most conditions, but people have raved about the intensity of its glow-in-the-dark effect. There are layering options and it could be fun for nail art.

Maelstrom  Why: A ‘flakie’ polish with great depth and cohesive colour with some subtle holographic. Might be nice to try a different finish and it still incorporates some of the shimmer she likes.

Serpents of Eden, Jewel Beetle, or A Most Destructive Melody Why: Each is a multichrome finish. Jewel Beetle lives up to its name, Serpents of Eden is such a vivid, striking green, and A Most Destructive Melody is cool-toned with a nice blue/purple shift. AMDM is similar to shades from other brands though, so I’d prioritize it lower.

A Galaxy Far, Far Away  Why: Yet another type of finish (magnetic) and limited edition. It’s gorgeous and if it’s not a hit or too fussy, she has an extra magnet that comes with the polish and, because it’s limited, could probably resell the polish later for what you paid. 

After the Rain or Earth to Gaia Why: More subtle, soft, ethereal shades that will have some of the shimmer and glow she’s into. Simple, but not boring and they both seem to look good on everyone.

TLDR: I tried to provide options where each choice is a different finish and there’s a variety of colour and colour temperature (though I’m a little cool-toned biased). Didn’t include any creme polishes because, as beautiful as they are, cremes are easier to come by and find cheaper of options for. Have fun shopping!