[Updated with new map design]
In the recent Brainstorm episode, the cast answered some questions from the General Questions topic. One question was if the Elemental Lords could be killed, and what would happen to the region. They said that the Elemental Lords could die, but didn't have anything thought up for the effects of the death. Here's my thoughts on the topic.
The current canon states that each region is sustained by its respective Elemental Lord. The Matoran can tap into elemental powers due to their access to the surrounding elementally charged land.
Taking this concept further could explain more aspects, like the layout of the island as a whole. Each region is a separate biome. These regions can have drastically different environments while still being in close proximity to each other. For example, Mangai and Ihu are stated to be separated by only a volcano and its rocky terrain. Yet the two regions are polar opposites, with the fire and ice remaining untouched by the neighboring biome. This is all thanks to the elemental energy flowing through the land from the Elemental Lords. It basically makes a sort of elemental force field to maintain the distinct biome.
So, if an Elemental Lord was killed, what would happen?
First, the elemental biome of the region would loose its power source. The elemental field protecting the region would be knocked offline and the region would eventually succumb to a climate change, due to the neighboring environments bleeding into it. However, this would likely take a very long time on its own.
Next, the Matoran lose their connection to their elemental powers. This will not be instantaneous. Each Matoran has a reserve of elemental energy that they tap into, with the more skilled Matoran having larger reserves. After prolonged use of their elemental powers, they get tired and must rest to refill these reserves. Their bodies take in elemental energy from the surrounding land to refuel them. However, if the region lost its power source, the Matoran recharging would only serve to deplete the remaining biome energy faster. And once it is gone, the Matoran would no longer have anything to refuel themselves. They would then become powerless.
This drying up of a biome would likely lead to the region becoming completely overtaken by its neighbors. The Matoran of the region would either evolve to tap into the neighboring biome's element, or they would die off due to the separation from their original element.
Here is a possible layout of each region with the potential effects an Elemental Lord dying.
If Mellum died, The heat of Mangai, Motara, and Naho would eventually melt the snows of Ihu. The region would become a barren mountainside with Naho's bay overflowing from the melted ice.
If Akida or Ketar died, then Tiro would eventually overtake them. The forest would flourish by overgrowing the beaches, or it would slowly cut through the desert without the sandstorms to keep it at bay.
If Uxar died, Kanae would eventually fall into the ocean. The other regions would not have much effect, due to Kanae's separate landmass.
If Terak died, the dessert would eventually consume the forest. Naho's rivers would not be enough to keep the trees alive amid the encroaching sandstorm. The heat from Mangai would also quicken this process.
If Ikir died, then the volcano would eventually become completely dormant. The chill of Ihu would eventually creep its way over the dead volcano and spread over Mangai. The sands of Motara and forests of Tiro would aslo start to creep up from the other side, without the super-heated air to turn them to ash. Eventually the three would meet in the middle of Mangai, fully assimilating a portion into each.