Tarluk
Terrarian
What Tripoli said, basically. A new engine alone will be able to open the door to many new features that just aren't possible in the current game; the devs have gone on record numerous times stating how the current engine has been limiting for them. We can only imagine how many features have ran through the developers' minds that they decided simply weren't possible without either switching the engine or drastically changing how the core mechanics of the current game work. Something like the islands suggestion I've seen a few times: being able to go to the Ocean to sail away toward an island with unique terrain, biomes or features simply wouldn't be possible within Terraria's current engine, for instance.
But does it really even need to have as much stuff in it as 1.4? Terraria's largely focused on quantity over quality when it's come to equippable items ever since 1.2 came out, creating continuous power creep in each and every update even for just a single point on the long line of progression. It used to be that beloved weapons such as Excalibur or Magical Harp were some of the best in the game, but nowadays they've been vastly overshadowed by weapons that are not only flat-out upgrades but are easily obtainable before you have the opportunity to craft either of those weapons.
It's something that I've actually quite disliked about recent updates, since in 1.0/1.1, sure, there's less items and less ways to obtain them, but that meant the items you did get felt so much more special, unique, and more impactful of an upgrade, and the game could tailor itself around this lower quantity of items and methods to obtain them. Nowadays, there's dozens of weapons and armor sets all competing with each other every step of a way; something like the Starfury felt absolutely awesome back then because there was truly nothing like it in the game, but now the Starfury is just part of the dime-a-dozen projectile melee collection, and you can most likely replace it within even just minutes of obtaining it.
I don't know that there will be something truly new that Terraria 1.x could not have done. Something worth investing time and effort into a game that by definition cannot have as much stuff in it as 1.4 would.
But does it really even need to have as much stuff in it as 1.4? Terraria's largely focused on quantity over quality when it's come to equippable items ever since 1.2 came out, creating continuous power creep in each and every update even for just a single point on the long line of progression. It used to be that beloved weapons such as Excalibur or Magical Harp were some of the best in the game, but nowadays they've been vastly overshadowed by weapons that are not only flat-out upgrades but are easily obtainable before you have the opportunity to craft either of those weapons.
It's something that I've actually quite disliked about recent updates, since in 1.0/1.1, sure, there's less items and less ways to obtain them, but that meant the items you did get felt so much more special, unique, and more impactful of an upgrade, and the game could tailor itself around this lower quantity of items and methods to obtain them. Nowadays, there's dozens of weapons and armor sets all competing with each other every step of a way; something like the Starfury felt absolutely awesome back then because there was truly nothing like it in the game, but now the Starfury is just part of the dime-a-dozen projectile melee collection, and you can most likely replace it within even just minutes of obtaining it.