Unexplored areas beyond 500 feet from explored areas will generate new biome terrain-related content when you are within sight of them.

New content that aren't biome terrain related, but appear only in specific types of biome like the Abomination have so far, been able to generate in explored areas.

For example, I surmise Mountain caves will not generate on explored mountains, because they are biome terrain specific. However, if IronGate introduces goats into mountains, the animals will generate on explored areas because they are not a part of the terrain, but entities on their own. Make sense?

On my dedicated server, we have a loose-rule, where players can explore beyond the islands directly around spawn, if all the areas around spawn have already been claimed enough. Therefore, there are still a lot of areas that are left to explore. However, I surmise by the time two biome contents are added to this game, we might just start a new server world.