I think a cool plugin for fire fighters would be a fire siren. Another idea is that someone would make some tools for fire-fighters (e.g. fire hose, fire extinguisher, fire engine, protective equipment for fire-fighters etc.). I think this would be very cool.
New items is impossible without client modifications. And what exactly do you mean by fire sirens, explain in detail?
well.... what you could do is when a fire is near a player(s) a sound can play through spout. the sound can be the firesiren you would hear at a firestation. When a fire is near the player the selected player will receive a water bucket ,leather armor suit , and ,maybe a stick or sometype of tool to hit the fire. the selected tool would have insta break to be able to get into the area of the fire... if you want i could make this. it would take a bit longer since i have school this week.
maybe you can make something like a noteblock with an iron block right next to it and when something gets on fire near it that block changes from iron to a redstone torch every like 2 seconds setting off the noteblock sorry if that doesn't make sense xD
You could right click when holding a bucket to launch a water projectile (with the projectile sprite being the water block) at the same angle but lower speed than arrows, and as soon as the projectile collides, a water block will be placed on the side the projectile landed. Of course the water bucket will change into a normal bucket after the water is launched.
I mean the kind of siren that you here at a firestation. In your server you can build a firestation (like me!). The selected fire fighter(s) can turn on the siren when someone said that there is a fire (first the siren has to be build with redstone repeater or something else). Than the firefighter(s) get spawned into the firestation (or the place where the fire broke out) and get(s) his/there armour. TopGear93's idea sounds very, very good. You could combinate it with my ideas. (I think it could be combinated with something like a vehicle plugin and that could be a fire engine)
so far ive got the main "structure" finished. im trying to add the little things that will make this interesting. so when i get home on my main pc ill work on this. But the vehicle idea wouldn't work that well unless you have tracks every where and homes near the ocean / water.