Thanks to everyone that replied with the feedback! I wasn't sure if I was going to continue this project but the positive reception has definitely made me re-think my decision.
I agree that one more weapon type should be plenty, although I haven't decided if I want it to be offensive or add some defensive elements to it. I'm definitely going to include more enemy types or at the very least have more complex patterns you must weave through, though I will also make the controls tighter if i do that since they are deliberately clumsy to add effect.
Any other suggestions are welcome. Should I have more than just the sperm theme? I was thinking for the second gameplay type maybe some kind of alien or zombie invasion (think space invaders but you are controlling the invaders, but instead of flying in space they're maybe walking down a street "Thriller" style
) where in the sperm levels you try to cause as many children to be spawned (twins, triplets, octo-mom?) that would provide more creatures (or hit points in a sense) for the following invasion level. That may be a bit much though...
A friend of mine suggested having controllable key shifts, or having splits in the path that would change the music with each new path, though both of these would be tricky as I don't know how to accomplish either.
Anyway, thanks again!