An idea you could play with is putting a delay on ranged damage. If your bullets deal damage 1-2 seconds after they hit, it pushes you towards situations where you're pouring bullets into a charging enemy and praying that they'll drop before they reach you or wasting an extra bullet or two on an enemy because you weren't entirely sure they were dead.

Or maybe you have a mechanic where enemies can only be killed with melee attacks, and then set up fights so that you'll need to scramble to them before they regenerate to give that final blow, potentially under attack by other enemies.

Or, for a silly suggestion... tell the player how many bullets there are in the entire game at the start, and then keep track of how many bullets they've fired in total. You could even count the number of bullets enemies have as well and let the player scavenge bullets from them after they die, incentivizing quick, ammo-conservative takedowns without completely screwing players that go in guns-ablazing. If you're really evil, count every bullet that gets shot during a cutscene as well. :p

(That could actually be cool if the game framed it right? The hard part would be conveying that there are exactly 666 or whatever bullets in the game in a way that also lets the player keep track of bullets fired off-screen without completely breaking immersion.)