I would LOVE an app for Terraria gameplay, especially with the recent update! I am thinking that it would be more on the database side, like your Metaforge app for ARC Raiders. Imagine using the wiki, but as an in-game overlay with item, world, and player scanning capability.
Scan items for recipes (dropped-by, materials + workstation to craft, used to craft, category, used-for, buy/sell for, best/worst modifiers, etc.)
Scan players for stats (health, mana, defense, loadouts + suggestions, etc.)
Scan worlds for stats and milestones (orbs smashed, meteors landed, invasions cleared, hardmode/pre-hardmode status, mob kills, boss progress, biome spread, NPC availability & happiness, etc.)
Additional
Resource tracking (bookmarked recipes)
Crafting Trees (progress tracking)
World Seed Information (popular/known predetermined seeds)