Considering it is the future of Terraria, I don't think there should be a sequel. This is because there isn't much of a story in the game, instead there are unlimited possibilities. But think if Minecraft, no comparison intended, were to have a sequel? It wouldn't and shouldn't work. Yes, the current Terraria is restricting the developers from more grander things, but it's strange.
Just my opinion and I wanted to get it out. Nonetheless, when the new game comes out, assuming it will, I will gladly buy it.