Instead of expanding the island, add new islands, as well as ports. Also, you could have a different kind of indigenous creatures. You could also make it harder to get the better weapons and increase stats, as well as adding more stats, like strength for melee weapons and acrobatics (allows you to move faster while jumping).
Henrique and halo are right saying to add headshots that are instant kills and to add rockets, you should also add bow and arrows and crossbows, and fix the sword. You could also add more quests and instead of adding to the island, make new islands with stronger monsters and a seaport (or a boat) on each one. You also need more guns, yet instead of adding stronger ones, just put in in-between guns or guns that are overall equal yet have different stats like more accuracy for this one but less damage than that one. You need different kinds of shotgun ammo too, like birdshot and buckshot as well as deer slugs. You could even add incendiary rounds that glow when fired and do DOT to enemies after hitting (with a fire effect so we know it sets them on fire.) Happy programming!
Since you're making more classes for dumbsaint, I have a few ideas... Assassin- Invisibility and Assassination abilities along with the grenades, guns are railgun and SMG. Gladiator- This is kind of dumb, but a more powerful version of the grenades, a battleaxe, and a rocket launcher. Last but not least, Tactical- A mix between tech and gunner, except more shooting than gadgets. Those are all the ideas I have. I'll be back with more.