Congrats on getting this out, I recall you started quite late, so what you have is accomplished in the short time is good. I really like your sprites, and got some vibes of an old game I used to play on either the nes or mega drive, though I can't remember what it was called (I'll have to go and figure that out now).
I'd echo some of the feedback you've already received, faster bullets, sfx, winning conditions and more variety of encounters etc... Another thing to consider would be power ups.
Hope you get time to work on this some more as it's a great start.