Simple: more types of bullets. Like for example, dum dum bullets (expanding bullets, causing more tissue damage but decreases penetration, at least, in case of hollow-point bullets).
For example, the Deagle could get one, seeing as it could be loaded with .357 Magnum cartbridges, and those cartbridges have a variation called JHP which are hollow-point bullets.
Of course, bullet-switching would get its own button, and to do so, you would have to wait a time equal to the double (or maybe 1.5x) of your weapon's reload time (only for balance purposes. Obviously, you don't have to reload your gun with bullet 'A' just to reload it once again with bullet 'B' in real life.).
These bullets could be found in the map, but they would be very rare or a bullet pickup would not provide too many bullets, only like 5 or 6.
In game example: Player 'A' with Deagle loaded with dum dum bullets shoots Player 'B' (100 HP, 0 Armour) once. It would deal 34 damage (base damage of Deagle, assuming perfect aim and constant damage) and then Player 'B' would start bleeding, that is, he starts loosing 1 health for 5 seconds or so. So he ends up with 66 Health after the first shot and 61 after the bleeding.
Bleeding could be stacked for longer durations, but medikits instantly stop bleedings.
This was just an example of what I want, not to be taken seriously.