ideas
bullet-repelling bullets: they rotate other bullets in a (very wide) range to move away from the point of impact when they impact
bullet-attracting bullets: same as bullet-repelling bullets, but attracts bullets
shockwave: knocks all enemies away from the point of impact (either only a little bit or with a maximum distance per time period
big bullet: bullet has wider radius
wind: a debuff combined with other benefits. ("shoot 6 wind bullets twice for 1 energy", "shoot a big piercing wind bullet for", "the next attack has wind and bounce and pierce", "the next attack is combined with a copy of it that has wind and bullet-attracting". knocks enemies back(a lot more then shockwave) instead of killing them. also flies much faster. When combined with love, the love only has a chance of 2.5% to apply.
repulse: AOE on impact. enemies move out of the AOE once before continuing to move towards the player.
scatter:makes bullets from the next attack have a slight randomness in their rotation
split: bullet turns into 2 bullets rotated 45° in opposite angles instead of disappearing, X times.
vibrate: makes bullets move sideways like a sine wave
portal: costs X. next attack you play has X less energy cost (minimum 0), but has a delay of 10 seconds
enchant: next attack improvement effect will apply to the next attack on the same card slot instead of the next attack
autoclicker: applies to the slot it is played on. automatically plays card (if there is enough energy) every 2sec on the mouse position specified when the card was played. if an autoclicker is already in play, the card is free, makes the autoclicker disappear and shoots 1 bullet.
discard: next card you play is free and doesn't have any effect
die: costs X. plays a random card that's not on your hand and costs X or less. unspent energy is regained. (if there is no card cheaper then or equally as expensive as the die, X random enemies die. joke feature maybe don't do that or make it a seperate card)
a card that lets you regain health: shoots 1 bullet for 2 energy. stacks on the card slot. (goes away when played on an other card slot or if an other card is played on this slot.) if it is stacked to 4, the stack disappears and you gain 1 heart.
structures: you can have a maximum of 4 structures in play, placed at mouse position. they are destroyed by enemies or by your own bullets. maybe they also time out?
turret: turrets fire an additional bullet each time you attack, but energy regenerates slower if there is at least 1 turret in play.
barrel: explodes when hit by your own bullet
regenerator: lets the player regain 1 heart every 3 minutes. player loses 1 heart if it is destroyed.
passive powerup that doubles the lifetime of bouncing bullets
passive powerup that makes explosions bigger
passive powerup that makes it so if you doubleclick a spot, all cards are aimed at that spot automatically (you don't have to aim anymore) until you doubleclick again
passive powerup that gives u 1 more hand card
passive scatter for every card
passive enchant for every card
passive 50% big chance