I just had a idea.
The land of Terraria is flat, ending with two oceans. Instead of just the oceans creating the barrier, how about the world you create is a planet? Walk far enough in one direction and you'll end up back where you started. The underworld? The center of the planet? At the very center is the core. It is minable, and creates the single most powerful tier in the game. Swords with a reach twice further than the fiery great sword. Pick axes that can explode-mine materials. One bar of "core material" will sell for 2 platinum. At the absolute center? A single, hyper-bright chest containing a "Planet Heart". Equip it, and you'll be able to do something I haven't figured out yet. And you can also build a rocket ship and go to the moon, where you can do things I haven't figured out yet either.
What do you think? It's a rough blueprint of the idea I had. Would it be practical?