- A Volcano Islands errupting during set times of the day with lava flowing, ash, posioness gas's and fireballs that could hit your ship whenit errupts! They would be Bigger Islands with more mystery like abandoned Ancient buildings with ancient treasure! They would be puzzles based islands with Traps!!
and Because i like this gamers idea so much i'm adding it into the concept @solidsnakemig said - Instead of the Hawaii route you could go all Krakatoa instead.
You have your rumbling, I would tie it to RNG with some modifications instead of a schedule personally, which would indicate a pending eruption. The eruption would last for several days, but instead of lava flows building up on the edges of the island, like Hawaii, explosions destroy parts of the island which exposes new secret areas if you are daring to brave the volcanic bombs, explosions, and of course the toxic clouds of volcanic ash and gas.
And instead of the eruption just stopping, there is a final cataclysmic explosion that can be heard across the entire map that destroys almost the entire island(red skys can be seen in the distance of where it is). If you wanted you could have the eruption plume visible from long distances like the skull clouds. Time is up to get your treasures and secrets.
Then the island slowly rebuilds itself and the cycle repeats itself. More complex than a lava flow that cools and then erodes away, but definitely more spectacular.
I mean, Come On
