I'm not sure if this has been suggested before, sorry if it has, but I couldn't find any post specifically about it, so here goes:
I suggest world generation to purposefully spawn some islands in the middle of deep ocean, completely separated from the rest of the world, containing new unique mini-biomes, which in turn, contain unique plants and animals.
These islands would not have the ability to spawn with a thingwall and thus can only be accessed by sailing there yourself, adding to the reward of progressing towards boats that have the ability to cross oceans.
I'd love to hear everyone's thoughts on this, as I love exploring the world myself.
Feel free to let your imagination flow and give some suggestions for the contents of these islands, I'll start with: Papaya trees.