The ending of Lost was almost much bigger than what audiences saw. Nothing was that different. The characters and island were always going to be what they ended up being. But, one big addition would have changed things significantly: a volcano.
Lost executive producers Damon Lindelof and Carlton Cuse told the story to Entertainment Weekly. The summary is that Lindelof and Cuse wanted some kind of visual identifier to bring together the idea that this island was like a cork on a bottle of evil. The symbol was going to be a volcano, and it would have been set up in the third to last episode. In that episode, where we learned the backstory of Jacob and the Man in Black, Jacob was going to throw the Man in Black into the volcano, turning him into the smoke monster that debuted in season one.
Then, in the series finale, Locke and Jack were going to fight on the volcano as it got ready to erupt—kind of a natural-disaster ticking clock, with tremors, lava, and, eventually, good triumphing over evil. Lost even set up the idea of the volcano being on the island some time prior, in a third season episode that featured a Dharma classroom. And yet, it ended up getting scrapped.
The reason is simple: money. Producers and executives realized that all the volcano effects and potential location filming were going to be way too expensive for them to handle, especially when another final season set, the temple, ended up being more pricey than expected. So, in the end, the very literal interpretation of the island as evil was cut out and things were left a little more ambiguous. Same ending, Jack vs. Locke fighting on a rocky area, but just no volcano.
Read the full story—and some other things Lost had to change due to production challenges—at the below link.