A wolrd has oceans! Its not very creative but i think adding ocean-like biomes can add alot of content to the game. It could vary from beatiful shallow coral-reefs to very deep dark oceans with mythical creatures, and hidden treasures or rare loot. This also opens the opporunity to add items for under-water breathing or under-water lighting. Also some kind of deep-ocean ores, subarines and boats could be added!
A very colored, shalow, under-water biome whith a wide variety of creatures, maybe even walking corals
A very dark and dangerous place, god knows what you could enounter here
This could be a stand alone biome, full of so-called "black smokers", but it would fit perfectly in the dark ocean biome.
Again not very creative, but again a very interesting concept i think.
A frozen, cold, misty, vast, flat biome, whith abandoned structures or castles scattered around the landscape. The entire biome is surrounded by mountains making is pretty hard to access. Only a handful creatures live here. At night theres a beatiful spectacle of nordic light, which maybe used for some magic stuff? Would be cool
This picture gave me the idea:
A HUGE floating mountain of ice even bigger under water, very rarely found in oceans carrying valuable resources. U can also build your house on it
A big snowy mountain whith its peak hidden in the clouds, with at the top "ice temples".
So thats about it I hope i gave u some more inspiration. Keep it up!