I was thinking about some sort of mega cards which are very strong but you can only take one of them throughout the whole game and they are pretty rare. For example:

• An extra life each round (not sure how overpowered that is)
• 3 random non-mega Cards (needs testing for the number)
• Bullets fly three Times as far (dangerous with selfharm)
• For every bullet you shoot, one is sent out in the opposite direction too (maybe too weak as it only really works with splitshot or quickburst)
• Nerf: Opponents movement controls are inverted (very funny)
• All your constructs are duplicated (Gives a use to the category)

Feel free to use these or change the numbers or just ignore the whole concept.

More cards:

• Boomerang: Bullets return at the end of their trajectory and go back to the point that they were sent out from in a straight line
• Cure: Remove all nerfs in your deck (and maybe immunity to future ones too)
• Slowdown Bullets: Bullets slow down the enemy if they are nearby
• Dash: dash by double tapping in any direction (with cooldown of course)

Some other things:

• Theres a little spelling mistake in the Jump Sustain card where it says Sustian
• For some cards it would be helpful to have a number, for example how much faster do you reload or move
• If youre already making a longer gamemode, you could also add a Quick game Mode where you get one mega and two normal cards at the beginning and only play to three points
• is there any problem with players taking the same card multiple times? That would be interesting
• I saw this at the PLAY Creative Gaming Festival, in case you didnt know your game was featured there or were unsure wether it would reach anyone through that
• Maybe add a way for customizing the controls and add Controller support