another idea: no chests/ammo/hp, but you get full health at the start of each stage. guns have 5-15 shots each but 1 in 3 enemies will drop their types gun (bit like hotline miami?), attacking without a gun does a crappy melee + but you can hold down click to throw a gun as a grenade. also some kind of progression you unlock the more you play, like access to harder areas, more skillful weapons / character types.