There is, or was, a site out there promoting a concept called the "Fire Hog".
Essentially, the Fire Hogs would be modified A-10s taken from USAF surplus stocks. Some of the modifications include a belly mounted retardant tank, a FLIR sensor on the nose and an option to refill its retardant tank in flight. Just exactly what sort of tanker aircraft would do this was not mentioned.
According to the information on the sites I saw, the USAF was not too keen on the idea of sparing its surplus aircraft for such purposes, so I don't know if the idea is still going or has been abandoned.
I tried to modify a 1/72 A-10 according to what was on the sit, but it didn't work out too well, maybe I'll try again sometime.
As for what I'd like to see modified for that sort of thing, the first image that hits my mind is a Nimrod ASW bird opening its doors over a fire.
On a smaller scale, perhaps a C-2 Greyhound with its cargo hold filled with a water tank that could simply be dumped through the back door.