Let's face it: Istaria is a BIG place, and even with the mob spawns, cities, towns, and various structures sprinkled around, it's still a big place.
So, here's an idea. It's a fluff idea, so naturally it goes after all the "show stopping" issues that need to be worked out.
Consider the implementation of static entities that react in a negative way to both mobs and PCs. For lack of a better word for them, I call them Environmental Hazards. When a mob/PC gets within their trigger range, they may or may not activate (each has a probability of firing), releasing some manner of damaging substance that is random in nature (it could be DoT, AoE, etc).
Some examples of a few Hazards:
Lava Bubble: They are found in volcanic zones, and look like a quivering dome of lava. When they fire, they shower the area with flame damage (AoE).
Deathweed: These are in swamps and jungles, and resemble a mutant plant with spore pods floating above on long stalks. When triggered they release a DoT cloud of poison gas.
Butcherstalk: Found in forests and grasslands, they are related to the Deathweed. When triggered, they fire piercing projectiles in all directions (AoE).
Teslacrystal: Found in the arctic zones, they resemble the large crystal formations there, but are distinguished by the dancing electrical discharges that cover their surfaces and jump from crystal to crystal. When triggered they release a directed bolt of lightning at the target, and very large ones release an electrical storm in an AoE discharge.
Corrupted Blightshard: (no offense to those who play on Blight Shard is meant) These look like the crystals found in the blight zones, but detonate (or discharge directly) a bolt of Blight energy.
Edit: Further, Environmental Hazards can be targeted by players. So it is possible to detonate one at distance if they are posing a hazard to free movement through an area. And since they can trigger because of damage, it would be possible to set off a chain reaction in a patch of Hazards by simply causing one to fire.