The question of longevity and replay value is very different if you're interested in playing with mods. There are already mod packs that do a fair job of populating the undeveloped regions of the map, adding new monsters and bosses, diablo style loot enchantments and creature abilities, character classes, magic systems. mobile bases, friendly npcs.

And that's just the sensible ones. On team stupid we have tower defense turrets, boat nuts and a fully functional monster truck.