1) A toggle in System > System Configuration > Sound Settings to alter the volume of Player Character (PC) voices that is completely separate from turning off NPC voices. This would include the ability to turn them off altogether.

2) In the same vein, a toggle to adjust just the Battle Voices that is separate from the other PC voices.
Explanation of 1 and 2: It would be nice if the current Voice volume slider was broken down into NPC Voices, PC Voices. If only the two existed, that would be fine for me, but it really is just the Battle Voices that get on my nerves, personally, so adding a Battle Voices slider would be ideal, leaving PC Voices to cover the laugh, cry, waa sounds and whatnot (which could still be adjusted or turned off altogether) and Battle Voices to cover all the grunts and weird sounds the characters make in combat. The NPC Voices would just cover all the rest of the in-game voices that are not made by PCs.
3) The ability to test the Battle Voices (all of the sounds) in the Character Creator before choosing a character's voice.
Explanation of 3: I have chosen poorly in the past and been forced to use Fantasia to change my voice after hearing it in combat. Starting out as a caster, one may not notice until switching jobs to tank, melee, or ranged dps that the voices can be obnoxious and repeat certain sounds (looking at you Miqo'te female #5). Fantasias are costly and in the Mogstore, and we are only given 1 free Fantasia after completing ARR's MSQs. Being forced to waste a Fantasia just to adjust a voice that is annoying is somewhat sad. Being able to test all the voices beforehand to know what we're really getting, would be ideal. It would be very new-player friendly as well.
I am sure I can come up with other suggestions but these are the main ones that I have been thinking over for some time, especially when making new characters and needing to go to Youtube videos about character race combat voices before I can even make a character properly to avoid having to Fantasia it later.

I love this game and thank all of those who created it for me to enjoy. Thanks for reading. Take care.