We can wait for new biomes while getting something spicy after beating all current bosses. How about increasing the world difficulty by spawning alot more of the 2-3 stars enemies counterparts?
Imagine walking back on the Black Forest and still having to keep your guard up because a horde of 3-star Brutes can still kill you, or having to fight for your life in the ocean against a 3-star serpent that can put up a fight against your late game ranged weapons?
And yeah.... Plains would be an actual nightmare (which I'd be fine with tbh)