Disclaimer: These comments are made using either real human proportions or TF2's art style as a basis. Whatever style you end up with, hopefully it'll still be relevant. I'm also going to explain things you may already know.
General comments -
When making drafts, you should start with ridiculously detailed sketches that accurately portray the volume of objects that you draw, taking into consideration things like perspective. I'm terrible at perspective in general, so I'm only going to comment on what jumps out at me.
2. Attention to detail:
When drawing from reference, it's important that you take note of things like placement and proportion. If you are going to exaggerate for the sake of caricature, then it's better to exaggerate in the direction that emphasizes contrast.
Pictures are nice, but what's even better is for you to get an actual object (or similar enough analogue) to play with so you can get a feel for what it's like. If you can't get someone to model for you, pose yourself against a mirror and take note of how your reflection looks.
Detailed feedback, from top to bottom -
1. The Head
* Exaggerate Medic's nonstandard skull shape, since that's something that makes him distinct.
* His hair curl starts in the middle of his forehead.
* The strap of the Blighted Beak goes behind the head like so: it goes straight back until where the ear meets the head, and then goes down and around. You are free to do whatever else on the details, but the function of the strap should stay as close to original as possible. Wear a headband to help you figure out how to draw it yourself.
* The ear should start at same height as the corner of the eye, rise up around the height of the brow, and then taper off at the height of where the lips meet. As far as how the ears are angled, anything goes.
* I tried this pose in the mirror. Once I turned my head so that the angle of my nose matched what you were suggesting with the angle of the Blighted Beak, I noticed that the front-facing portion of my face was not as wide as your drawing. The eye-holes should be further to the right, spaced closer together, and jutting "out" more. (While your character isn't posed in as extreme an angle as the reference included in this redline, it's still helpful to look out.)
* Exaggerate Medic's Manry Manjaw of Manriness. All classes with visible faces have longer chins and greater under-bites than the average human.
* Does not look anything like a dove, not even a cartoon one. Unfortunately, most clip art only feature doves in flight, but I was able to find pictures of 白鴿牌洗衣精 (White Dove Brand Laundry Detergent), whose logo is that of a brooding dove holding a single white feather, and I would suggest you use that as a basis for a more stylized depiction.
* Looks a bit too narrow compared to references, even taking perspective into consideration.
* Collar is inaccurate.
* Placing of visible portions of Medigun Backpack is too high. (In the picture on the right, it has the illusion of being higher because both the Medic's head and shoulder are tilted down. The smaller picture to the left is how much of the backpack is visible in front view.)
* The right arm (our left) is too skinny, and the elbow placed a little bit too low.
* Both wrists are too skinny.
* The left forearm (our right) should still be fully visible and the upper arm partially visible with this pose. Foreshortening should take into account Medic's thick rubber gloves, which are wider at the bottom than at the wrist.
* Fingers should be longer and thicker given the Medic's gloves.
* Buttons and straps are inaccurate compared to the original.
7. Rest of the body
* Should have at least been present in drafting stage. The artist is not a camera, and thus is not obligated to draw everything "in frame", but it's better to draw more and then cut away in editing than to leave things out and then have to add things back in later.
* This is more of a personal preference, but I think you should have at least drawn as far as the Medic's entire torso, or re-framed the picture with an even narrower field of view.