You got amazing backgrounds and see no reason why character drawings can't also be equally good.
Samus could have used a bit more detail.
Perhaps, you can look into more reference material of samus for your drawings.
I recommend that you study more already existing images of samus and carefully observe the details.
I seen a lot of chicks dressup as samus on the internet and many of these girls donned accurate costumes.
You have a lot of reference material to work from.
You are now ready for the next step after teaching yourself how to draw a more detailed samus.
The next step is building onto your drawings for a more original look.
Take your time with working out the poses of your animation.
Some artists form a skeleton of thier drawings before building onto it.
Others draw straight ahead and include the details too.
You can plan out the animation poses by acting them out on video tape, use stick figures first or whatever else.
Just work out the animation poses to the best of your ability.
You mentioned a low frame rate in a previous review response.
Strong poses are needed to compensate for a lowered frame rate.
Harofreak once taught me; "It's not the framerate but how you draw it."
That punchline about you not playing metroid has taught me something.
You never played any of the metroid games.
Your imagination can easily run wild and fill in the gaps.
That concept sounds like it has a lot of potential.
I look forward to your future movies and possiable orginal ideas too.
-Mightydein