What caught my eye was the game thumbnail, because the character looked cute. I think it would be cute if the camera was more zoomed in on the character, so the player can better view the character and their details since that can better hold the player's attention.
From a gameplay perspective I think a check-point halfway through would have been nice. I think the freefall with the spikes area was particularly difficult and maybe a bit more space between the gap would have made it more fair.
I understand the background is not yours, but if you keep it I'd recommend maybe redesigning the platforms since they kind of blend into the background. Maybe make them bigger and more distinct in appearance would help.
The fireballs were interesting. I think they should start off slow in the early game and speed up as you progress through the game. I'd recommend that but after check points are put in of course otherwise the surprise fire might seem too unfair. Maybe for some fireballs you can have them be multi-directional like they bounce off the walls or they home into a spot you were standing just a few seconds ago if your character is idling.
It may be early development, but I feel adding some music and maybe set up a short intro and apparent goal for the player would go a long way even if they're very simple or don't reflect the final product. Having place-holders for music and story just like the background can help a game appear more engaging and can better help play-testers determine what kind of game you have in mind when it comes to theme and mood for when suggestions are made. Just having that can make a game feel more full despite how minor they may be or if they serve entirely as a placeholders in this stage of development.
Lastly giving the character a unique trait that helps out gameplay-wise would really help this game stand out, and I think less people would criticize it as "just another platformer game" if that trait or ability was creative and innovative enough. Maybe your up-and-coming magi has the ability to transform certain objects or obstacles into something else that aides her in game progression or maybe she's cursed so that sources of fire are always attracted to her, but this curse can also be used to her advantage as well to light torches or interact with her environment using the fire. I'd suggest something easy since this seems like a first time project, but I think it's good food for thought.
Does your character have a name by the way? It would be nice if the player knew more about her and what her goals/objectives are. Sorry if I'm asking too much when the game is still in early development. I hope this project of yours turns out well and that this comment was insightful for you. Good luck!