*Is there a semifunctional version of the game that I can look at to get an idea of where you are, and what you need other than character sprites?
there is the repo of the game on github you can download and see (which is currently using your sprites given)
*What about environments? Do you do one image, layered two-dimensional images, or tile-based maps? (or other?)
i dont believe this is figured out yet.
*Diagonal sprites. Do you need them? Could you use them?
if an image can be rotated, no we dont need them as pygame has methods to rotate images. Then we just have one image and can spin it 360 degrees and way needed.
*What dimensions should the sprites for the projectiles be?
i would assume something relevant to the size of the pea shooters mouth, maybe a hair bigger. But we can resize the image in pygame anyways
*Are buttons separate graphics? For example, would the "Start Game" etc.
The currentl placeholder is currently using text, but i think graghic buttons would look better
A lot of these question have yet to be discussed until now. So it is all up in the air. I think we would program it based on what the graphics are.