The Logo's presented in the beginning first appear clean and then with dramatic effect are "corrupted". They turn purple with veins etc. Then we are given quick flashes of both 2d and 3d images that give the impression of a 2d game world view but with 3d effects. Also, we are told that it isn't something they have mentioned in the past. Logically, I would assume that it's something related to Terraria because of those themes being teased.
Also, why would it be illogical to believe that they decided to remake Terraria? I mean, many developers in the past have done so with great success. It's actually a great way to welcome the next installment of a game by showcasing a new engine and/or effects through the original.
Again, i'm not 100% sure, it can easily be something entirely different.