There's already unique sound effects for each weapon, and sound effects for when your being shot at in the game. In most circumstances I think that sniper fire is not likely to be as big an issue as people are making it out to be, as sniper rifles use bullet physics, travel time, drop off, etc. Such that shooting someone from across the map would largely hinge on either accurately predicting where their going to be or if their standing still, and even then unless they score a perfect headshot with a high powered rifle it likely won't kill them, which makes such engagements more dangerous in mid range rather than long. In most cases you'd know your being shot at and either take or build cover, or start evasive action by jumping and zigzagging like a deranged lunatic, much like all users do in such circumstances. Generally experienced players tend to not stand still in the open as they've grown accustomed to the uncertainty of potential snipers lurking on the proverbial grassy knolls in the area, though whether they decide to build cover and give themselves away plays into an entirely different sort of metagame.

@ianhamilton_
Its difficult to say how effective a sonifier would be due to one never having been implemented in such a way, it wouldn't likely be perfect, but I think adding another layer of feedback about the environment in addition to audio cues would be quite helpful given the challenges involved. Also keep in mind that the sonifier's playback can be increased to fractions of a second for faster feedback rate, improving potential reaction time for users. I imagine a player could use quick scans for getting a general view of the environment when navigating and using audio cues to listen for any players nearby, or if they happen to suspect anyone they could quickly switch to a "scan only for players" mode for anyone that may be hidding or lurking in their direction, locking on and engaging with auto-aiming if required. Although something else to consider would be if locking on to a player in view could also be a way to detect players in and of itself with its own audio cue, I know the console versions have such an auto-aiming feature but don't think the PC version does.

I've also taken some time to update my Audiocraft prototype [here] with a new optimized sonifier with filtering capabilities, speed adjustment, and looping, which may help give a better idea of what such a system might be like.

@17
redspot, RTR, the killer, firefight and...
there are already a lot of FPS games and i think  DEVS can  add a battle royale mode. it won't be too hard for them to do it