So we have a cover where everything is nearly there, but needs some adjustment to quite reach the right standard.
Tonal values and colour
First of all, I've taken a pass for contrast and saturation. You might think the version on the right looks too much now, but bear with me!
I have overall heightened the contrast, making the highlights brighter and the shadows richer. Then I've particularly adjusted specific areas:
I've taken lightness out of the very edges so the eye is drawn to the middle better.
I've upped the contrast where the blue sparkle surrounds and crosses the figure. It's a really important element: it it the only direct hint on your conver of the fantasy nature of this story, and visually it draws the eye to the right places. You need to spot that sparkle straight away.
I've darkened the area behind the byline significantly. Ideally I would adjust the illustration so this deocrative border makes a frame for the byline to sit within. But if that's not possible, it certainly needs to be darkened. In aaddition having a dark area at the bottom edge helps create a range of tones across the cover which the current version is lacking.
I've upped the contrast of the sky to turn it from something that looks duly overcast to something that looks stormy.
I've particularly looked at the sky behind the brooch device, making the blue more intense to punch out the yellows there.
I've pushed the contrast and colour intensity up to a point where it sits a little uncomfortably because it's a good way to spot the problems you might be solving with fading it all out.
E.g. the adjusted version certainly looks a little too busy and eye-confusing now. But that's not because the colours are too intense - it's because the colours aren't harmonising.
Let's take a look at how things look with a few different colour tints:
Suddenly the colours don't look so OTT because they are toning together.
You'll also spot that these convey subtly different ideas of genre and mood.
The blue cast on the top left feels more wild and free than it's red-tint neighbour, which connotes something more grand and traditional.
The yellow green at the bottom goes hardest on the Irish/Celtic feel.
The author will know best which vibe matches the best book. My feeling, based on the information I have, would be the top-left blue shade. It balances richness and atmosphere with liveliness.
Now if I compare this option with the original cover...
... Hopefully you'll see how there's mood and atmosphere missing from the original. It looks muddy both literally and in vibe.
Now getting onto the issues it's less easy to demonstrate visually without having the separate eleemts of the cover to play with...
There is an issue with the texture/styling of different parts of the cover not matching in vibe/styling. The 3D shininess of the title and decorative border treatment looks artificial and smooth in a way that doesn't match the painting-esque styling of the illustration.
Playing with the intensity as above actually does a lot to help them all tone better together.
On the original cover the colours are washed out enough to look watercoloury and that sits oddly against a 3D gold effect. With the colours and shade intensified the painting looks more oil-painting-esque and doesn't jibe so oddly with the gold effects.
But ideally I'd still go for a gold effect better chosen to look as antique as the painting if you can.
It will also help the title look a little more bespoke and eye-catchingly weighty, to have a little texture going on.
The title font is well chosen and nicely composed as a title lockup. My one nitpick is that short upper bar of the 'e' of 'Legacy' throws the kerning off in that line and catches weirdly on the eye.
It makes me watch to read the word like it's French: "Le Gacy...".
But other than that the title treatment is good. The series-name and byline need looking at though.
They shouldn't be in the same (or this very similar) font to the title. They should be in a font which is less fancy but matches the mood in less attention-grabbing form. So, serif, classical, grand, capitalised. I'd suggest good ol' Trajan.
At present you're creating a problem in the visual hierarchy: the eye doesn't settle naturally on the elements in the order it should. As you read the title, the other busy blocks of lettering are pulling at the eye. The matching lettering and colour gives them the feeling they're all pat of the same element and it disrupts the natural flow of the cover.
Nathan is right, the size of the illustration could stand to come up. You have a decent arrangement at present but not I think the best possible one.
the top half is feeling a little uncomfortably cluttered, with the border, brooch and title all crashing each other a bit.
Meanwhile your illustration isn't really leaping out. At thumbnail this is how things look:
As you can see my adjustments have made things look more sharp and rich at thumbnail size and do help bring out the figure more clearly, but they don't solve the issue that the nature/subject/genre/title of this book aren't very clear at this size (and are still a little hazy at full size).
It's always important to make sure a title is totally legible at thumbnail. But in your case your title gives some info but isn't enough in itself to get across what kind of book this is. You'll also need to make sure the illustration is pretty clear.
At the moment you'd probablyspot the broad shape of the figure but she looks like she might be painting at an easle or reaching out to touch a mirror or a lot of other things. You need to have this image of the woman reaching out to the standing stone, and the fae sparkle around her, read clearly.
Making that larger will change how things overlap and cross etc. But I would like to add one inal note, which is to look out for what parts o the illustration are disappearing under text. Like Nathan says, it;s fine for the woman's dress to have text overalyed - it;s not an important part of the illustration to keep visible.
But at present you have text going over the top of your standing stone and I think that's a mistake. You might not be able to avoid having some overlap of text/stone as it;s the tallest part of the illustration. But the top itself should be poking out and visible. Your cover needs to frame the standing stone as important; it's the thing that tells us what this book is about. Currently your composition undermines the stone and therefore the book itself!
I'd be tempted to do something more deliberately Irish or fae within the cover. The image of a cloaked woman reaching out to a standing stone will immediately remind an awful lot of readers of Outlander. It might therefore be worth stressing the stuff which shows this isn't an Outlander ripoff, or you might well have people dismissing your book because they're not interest in Scottish timeslip romance - which of course is not what your book is, so that would be a shame.