I've always believed that arsonists should be charged with every single incident that occurs in a fire. It's a pretty straightforward approach.
10 people get burned to death = 10 charges of manslaughter 100 homes get burned = 100 charges of arson 10000 kangaroos or koalas, parrots etc get burned (or any wildlife) = 10000 charges of animal cruelty Damage to 1000 properties = 1000 charges of malicious damage
The law would specify all sentences have to be served consecutively, not concurrently. So each sentence has to be served separately, one after the other.
The end result would be sentences of more than 500 years for a major fire causing death. I'd like to see the parole board find a way to let an arsonist out with this approach.