The first thing I think if I look at it, is that the design is pretty bland and the pose is rather stiff.
Let me first talk about the pose and the proportions.
The main problem with the pose is that the knight is just standing, neither sword nor shield are with this pose ready for attack or block and the weight distribution which is 50/50 on both feet don't allows any quick movement out of this pose.
Knights were professional trained warriors, a wrong stance could be a matter of life and death and that's why they would never stand like that with an unsheated sword.
Proportion wise the helemet is pretty big, and the lower legs are really long, while the arms are a tad to short.
My suggestion would be to go with a more ready to attack pose with an actual weight distribution and carefully checking out all proportions:
THis edit illustrates where your drawing currently suffers the most.
Now let me talk about the design.
Designwise I can see that you don't know how armor works. The main goal of any armor is to protect the critical spots of the human body, while maintaining a high amount of movability and not having too much weight. There are tons of great full-plate examples from history around, displaying the craftmanship of the smiths and you also can see the differences in designs to protect the wearer from the most lethal weapons used in different periods of times. If you are going to make a game about knights, don't go first with what you imagine a knight looks like. Look it up, try to understand it and then you can alter those designs in order to get something which is fitting for your purposes and visually pleasing.
Your smallest problem currently, is that your metal doesn't look like metal at all. The only thing which will help you there is to understand how metal behaves in different lighting situations. This needs to be practiced with references. But even if you would apply a perfect metal shading to your current drawing, the difference wouldn't be that big.
to show you how I'd go over designing and shading an armor - 7 values used (changing the design would still have a bigger impact on your drawing).
To be completely honest, knights are really an advanced subject to draw, you need a lot of knowledge regarding basic art principles, the human body, plus a big background of medieval weapons & armory and how to shade metal.
Despite I don't even went really deep in the design aspects, my version still took me 3 hours to draw.
In order to get a complete game done, you might be better off if you significantly decrease the resolution.
