I've had an idea for an additional aiming mode that could be added via mods: heart shots, which are a type of shot that real-world world snipers might choose to use either when they're at extreme range, or when immediate incapacitation isn't critical.
It's basically just the concept of taking extra time to aim. To execute one, your character must spend one or more turns aiming. If they have at least enough AP left in a turn fire their weapon normally, and are in a single-fire weapon mode, they can press the "Heart Shot" button on their UI and select a target. This essentially begins the process of aiming (charging) a heart shot; mechanically, it consumes all your AP and starts tallying them. Once locked into a heart shot, the character cannot move or change targets without either canceling out of the heart shot or taking the shot. Subsequent turns may be spent adding more charge time (AP) to the heart shot by pressing the Heart Shot button again, adding that turn's AP to the tally. For every multiple of the amount of AP needed to fire the weapon normally consumed, the heart shot gains a charge, possibly reflected by a buff icon on the UI. Actually taking the heart shot also requires expending the standard AP cost of firing the weapon and confers no charges or bonuses.
With one charge, you'd get 2x crit rate and 2x crit damage; with two, you'd get 2.5x crit rate, 2.5x crit damage, and +5% to-hit; and with three charges, you'd get 3.0x crit rate, 3.0x crit damage, and +10% to-hit. Three charges is the max, though you could continue to hold the Heart Shot and waste subsequent turns/AP (to wait for a better opportunity, for example). Using a weapon that costs 6 AP to fire, three Heart Shot charges and taking the shot would require a total of 24 AP.
Of course, the fact that enemies usually tear across the map would mean you'd typically only want a single charge, or possibly two, but it still seems like a cool option to me.
If you hit but don't crit despite your heart shot charges, it means you hit the chest but not the heart or other vital organ.
While this takes time to explain properly, I'd think it would be fairly easy to both implement, and understand and utilize.