The Terraria developers don't believe in paid dlc's or content updates. What they do is free updates that can add pretty much anything, including new bosses. Not very many developers do that which make them pretty singular in that respect.
If you're thinking about mods (the pc version has a thriving mod community), that will never reach the console or mobile versions of the game. The console manufacturers don't allow it. You may argue that Bethesda does it (with Fallout and Skyrim), but mods work very differently there; they are run in a sandbox environment which limit what a mod can do. They aren't allowed to modify the game's code. Terraria mods on pc have free reins and can do pretty much anything as they modify the game's code directly.