I came up with a similar thread a while back that involved possible boss implementation - the replies are more interesting than the concepts I proposed: http://www.graalians.com/forums/showthread.php?t=19856

I agree with this to some extent due to the lag it may cause + scripting complications. That is why Graal Classic is subtly releasing these NPC's into the world, but on a much smaller scale. (examples include bandit leader and flaming blob)

The weather system is a good idea, but shouldn't serve as an indicator for boss spawns. It should remain aesthetic so that players with older devices are not left at a disadvantage. Off-topic, but what Graal Classic needs is a day & night system.