Personally, on the subject of mods that fit in with vanilla Terraria, and feel like they could be added, I think none of the big mods do.(Thorium, Spirit, Calamity, Tremor)
In a gameplay perspective, perhaps. However, items tend to be to unique early on compared to vanilla, and spritewise are more detailed. I'm excluding Tremor and Spirit from this thought, because I'm not quite sure/remembering how the early game is "supposedly" gonna be with Spirit, and Tremor actually somewhat maybe does, just based on my memory.
However, one big problem with these mods - Bosses can't feel like their important to fight, unless past Moon Lord, since you can skip them and go through the game fine. All/most bosses in Terraria at least feel like fighting them progresses the game - The few that don't I do tend to not care to fight, even though in mods I do want to fight them. And it is impossible/incredibly hard to make them feel like progress in the game if you mod them in, I do understand that. Post Moon Lord is different in this regard, though, but only one has those - Calamity.
However, of any mods that do feel like they could be an official expansion, I'd say the order goes Thorium, for being a super awesome mod that fits in quite well, to Spirit, for seeming like a super awesome mod that fits in quite well, to Calamity, which I say is lower then those two for being harder with bosses then Terraria, and the sprites aren't quite as Terraria, and then Tremor, for reasons.