Potatostrike is definitely going to be a commitment, but I think the potential to do well might be high. Glad you are considering it.
As for BG, I think you have some bg that can move up to earlier that already exist like the pan with the mountain in the later side of the campaign.
Projectile fireworks, a good visual example would be Touhou series, the whole screen is just fireworks over and over, different shapes and color combos, lots of circular forms. I heard other devs have better luck with fiverr for art commissions if you ever need to try for that.