“Like your shiny new Cobalt, but want to add some additional flair? Well, why not get this card foil, and add rainbow tracers to your bullet!”
This is just an idea that I had came up with. Card sleeves would only be obtainable with real currency, but would add nice additional effects to the merc that it is applied. You can also swap foils across different cards, but you cannot use one sleeve for more than one card at the same time (so, to cover 2 cards of the same sleeve, you will need to buy 2 sleeves of the same kind). Of course, these special effects aren’t just limited to bullet tracers, but can possibly extend to masks, face paint, tattoo, etc., and maybe even hats! 