Surely any likely 172 replacement (Cessna Caravan,Defender 2000/4000, VFW260) would be capable of carrying a for want of the correct technical term, bucket.
What about Light twins and medium lift aircraft could they lift and safely carry the required load at the correct altitude.
I see Celtic helicopters have been using Bell 206Bs and BO 105s for recent firefighting operations so I don't see why a light twin utility couldn't.