When Re-Logic announced a follow up to Terraria last month, the studio stopped short of explaining what it actually is. But now we're in the future, a pleasant future bearing details on Terraria: Otherworld, and the gist of these details is that Otherworld adds "strategy and roleplaying" elements to the pixel art sandbox, as well as "purpose", in case building things for no reason isn't your bag.
"Set in an alternate dimension within the Terraria universe, Terraria: Otherworld places the player in a life-and-death struggle to restore a once-pristine world," the update reads. "Along with a rag-tag band of survivors, will you be able to successfully harness the power of an array of weaponry, magic, defenses, and even the world itself to thwart the designs of this unseen evil?"
For every life and death struggle comes the inevitable levelling requirements, and Terraria: Otherworld will add them in spades while still retaining the open-ended nature of the original. That means the story-focused elements won't be linear, the NPCs will do more than just sell stuff, and skill trees are in.
On the topic of "purpose", you'll be tasked with "pushing back the Corruption until it is no more". You'll do this by finding or crafting Purifying Towers and then activating them. Once activated, you'll need to defend them also. That activity forms the basis of the story-driven game, but if all you want to do is explore and build, it sounds like you'll be free to do that, too.
The full details are on the Terraria forum, but no release date beyond a vague 2015 window has been announced.