Nothing wrong with being influenced by great characters but in my opinion you need to push the design further away from those homages...unless you're specifically trying to design the character as a pastiche.
I'd say I did that though. You can tell the influences there, but at the same time, she looks pretty distinct from the two in my honest opinion.
Not trying to talk you out of your opinion but how about a little constructive exercise ... If you're game...
Walk me through the visual elements in your design that I can't also assign to a version of one of the two homages characters.
- Her pale skin and black/purple eyes.
- Her "urban sorceress" look. Scarlet Witch looks like a, well, witch, and Zatanna a magician.
- Her bustier.
- Her style of hair.
- Her boots.
These are all different from both to me.
That's more a Raven thing but as trope, it will get noted if you don't hard left from homages
The MCU version veers to the urban approach ...complete with bustier.
Perez did the belly cut bustier/corset look alot. He combines it with Gypsy cut skirts for a more old world. feel.
Zantanna wears a variety of boots, including the thigh highs, depending on the artist.
I like how you drew the hair...but with out other examples of you drawing long black hair, I can't tell if it's a style or stylistic.
I like the drawing, I just think the visual elements don't move far enough away from the sources you're citing.
I just feel like the design is unique by having all these different elements and the way it's presented. How do I even move far away to avoid comparisons while still being aesthetically pleasing? Some things just unavoidable.
Now, come on =) The girl does look like these two (and I hope the author will make some changes, because her look is as impractical as it gets for starters), but you can approach nearly every character's design like this: oh, I saw it here, here and here. Well, of course you did, corsets and high boots and even those eyes are nothing unique and were not invented in Marvel too.
I understand that but when you're designing a witch/sorceress ...it's problematic to have most the visual elements similar two (or three) iconic witch/sorceress characters.
Perhaps push the non-human vibe a bit. She can remain human-like, but her outfit can show her non-human characteristics.
You can keep the urban style and add some symbols and "alien-like" items.
Example: the shoulder pads giving off the "ufo" vibe. It might be far from the type of 'non-human being' you're going for, but you can see here how to break the uniformity of your urban outfit with some weird designs. David Bowie is the King of such successes btw. Looking up his stage outfits often gives me inspiration 
The textures could also reflect it.
Example, keep the shape of that coat but have it made of algae or weird organisms that look alive. Something resembling such textures, perhaps?
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And in the end, the outfit may even grow out of its "outfit" status and give your character some more depth regarding her magic abilities or stuff.
Example: the algae coat could come alive when she's casting spells, or shapeshift slightly, or change colors, or turn into rock, or spew butterflies as she runs away, etc.
Those are mere examples of what you can do to tweak the overall style of your outfit without losing the urban witch vibe you initially wanted. This way, the outfit can be born again with new characteristics and items without losing its source, and you get something more original and tailored for the MC and her backstory.
Anything can be done to tweak an outfit. Browse pinterest for ideas, add stuff you think haven't been used yet, or could be used more efficiently (especially regarding textures, articulations of elements, colors), make brand-new stuff with old school vibes, have fun
Once again, think David Bowie. This guy saved my inspiration.
So if I got rid of the lacing on the bustier and the boots, and changed the pantyhose to sheer, would that make the difference for the artist?
Everyone's got the design elements covered.
I thought I might add that while I do like the color combo, and I think the art is pretty, I find all the elements are not very distinct from one another, and the whole outfit blends in with other elements, in spite of the vastly different colors. I think it's how the shadows are done.
I really like her hair and how it's swirly :3
Sure, she would be easier to draw at least. Characters with elaborate design are taking a hell of a time to draw, look at the original Scarlet Witch costume, it's very plain. Zatanna's a bit harder, but it's very plain corset, plain boots, plain trousers/panties. Just think about design a bit more 
I have a question, what do you think of the design regarding its own merit?
For my world, it's a fantasy kitchen sink like Marvel/DC. I've laid out a system of how different characters look, and how far I can take it while maintaining internal logic. It's fun to see how different I can make characters look while adhering to it.
Here it goes:
- Peak humans and physical normals dress in the most practical manner possible, favoring gear and clothes that benefit them in combat.
- Speedsters wear running shoes and often have goggles just to make sure their eyes are alright.
- Mages (like the one above) dress in fancier and more stylish clothing, as it befits them and they can manage it.
- Tech-users often have, well, technology on them, ranging from power armor to various gadgets.
- Those with greater powers often wear clothes the lower ones couldn't do, such as more elaborate costumes for the men and sexy costumes for the ladies.
I love costumes that are cool and/or sexy, but I also like it to fit each character as something they would wear and can reasonable do so.
For a main character her design is a bit complex. As a main character, she's going to be drawn a lot, so ideally you'll want to keep it simple for the artist's benefit.
There's a lot of Scarlet Witch in her with the colors and hair and how her energy works. That's not a bad thing, but people will keep comparing her to the Scarlet Witch. Maybe give her a different color? Green maybe?
The interdimensional being thing doesn't really come through. Maybe change her to look a bit more like a Dr. Strange character? It's hard to have an interdimensional being dress like an urban wizard and still get the interdimensional thing across because there's tension between those concepts. Urban means mundane, city clothes, and that's not exactly alien.
What's she like personality wise? Zatanna has the magic performer thing and the Zatara legacy thing. Wanda has hard to control chaos magic and a million love triangles. What's Astra's deal?
I think I agree with others' suggestions of simplification.
But one other thing I would like to mention is function. The way she dresses should be practical either to her role in the story or her character.
For example, wearing heels makes it harder for her to move around and run. Shorter heels might be more believable, or at least something that looks easier to move in - these ones would restrict the knees. It's possible for knee high boots to look more practical, like the ones you see in Attack On Titan for example. If there's a character-based reason why she wears those, like an emotional attachment to them, that would be an alternate function.
Maybe adding some elements that actually indicate her being a mage would help, too. I can tell you're going for a gothic style of dress so maybe something with runes on to tick both boxes. Runes are also easy to draw. You could put runes on her coat belt, her skirt... literally anywhere. You could even replace those bothersome laces with straps that have runes on them. Those would be far easier to draw and also more functional to the design.
I mean, Zatanna has been included here and everything she's wearing indicates her role as a magician - she looks like a showman from top to bottom. Also, she looks simple enough to be reproducable for multiple panels.
The wand is a good start, and so is the long coat for that mage vibe. However, it still feels like things are missing.
I think the skirt feels a bit randomly placed, too. While her boots and tube top have a kind of whimsical look to them as well as looking fashionable, the skirt just doesn't give off a vibe of magic or fashion. A good replacement would be a pencil skirt (a nice stretchy one to allow for movement) or maybe a shorter skirt with spat shorts underneath (more fashionable and modern, while also being easy to move around in). The skirt she has looks like it would get in the way of her legs while also looking very out-of-fashion.
This is definitely a strong start, though. You've got a tonne of great ideas going on. It just needs those slight tweaks to make it look more unique and memorable. You should be able to look at this character and say "ah yes, that's Astra!".
As for making her more memorable, maybe you could add some intrigue to her hair. It's both fashionable and practical to style your hair if it's long. She could wear it in a ponytail, or even have some braids to enhance her "mage" image. Braids are very whimsical and witchy. You could even dye streaks or other colours into her hair to add a quick and easy element of fashion to it.
Look into some witchy and high fantasy elements you could add that are unique or less common. That might be a good place to find inspiration. Look into things like wicca or hellenistic witchcraft, since they're more modern and an "urban sorceress" is more likely to look a little wiccan or hellenistic. Even looking up things on pinterest will give you plenty of image inspiration. Type in something like "witch" or "wicca" or "sorceress" to get some quick and simple visual stimulus. That's how I get my brain going when I'm designing a character. If you want, make a pinterest board of character inspiration. I have character boards that I use when drawing mine.
Judging from previous responses, I don't think OP actually wants constructive feedback and just wanted to show off (the art they commissioned) and maybe tweak small things on the design. Not change entire things like what shoes she's wearing. Feedback can be a harsh bitter thing to swallow sometimes and for some people.
Take for example the design of Rita. Her colors are harmonious, like OP's. But she's wearing something that fits her behavior and practicality. She studies a lot, so she carries a book at her hip where the weight can be balanced by her own hips. Her tunic is short and designed somewhat like a kimono, leaving her obi/sash to be her literal weapon for physical combat when spells aren't enough. She has open spots of skin at her upper thigh before her leggings, and then she's wearing flat boots. A pretty acceptable design, and then you're told she's about 15. I'm not saying design everything to be a-symetical or odd, but different elements of her outfit has different trim and coloring. Her clothes can be practical for both lazing around her house in mountains of books, or walking through ruins, or in battle.
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You want a long haired mage that retains more modern idea of colorings for mages? Magilou is a circus magician who can do actual magic. Her outfit reflects that.
OP wants a mage with traditional mage colors and long black hair?
You want one dressed in red?
You need to accept as a creator that some designs are just used a lot. Corset, thigh-highs, skirt, trench coat, with two colors and no unique trim or design elements to make it stand out and the character blends into the crowd. You can alter the actual clothing by still keeping the silhouette. Take Velvet as another example. She wears a trench and has lace and is two colors. Her design isn't practical but it's not meant to be - her design is specifically after she's been locked in a cage and forced to be a beast for several years. Even her long black hair in it's braid is all messy. Nothing about her is refined or sweet.
I mentioned my actual feedback before with questions- WHY do you have to have your character in a trench? In a corset? In a skirt? In tall boots? Honestly, it looks like a nice design, but you're automatically aging her design to something that either is punk rocker, or 80's ish or 90's. Some of those designs from google I pointed out have small things that make them unique - specifically that last one - they're dressed in armor that has trim to it so their black outfit shows that they have boots and a shape under her red jacket.