I've got a thing for the 'Natsu returns stronger' trope, personally. After Tartaros, he vanishes, maybe training with Ignia or something, and comes back just... different. Not colder, exactly, but sharper. The way Lucy navigates that change, trying to reach the boy she knew while accepting the man he's become—it gets me every time. I'm picky about it, though; I hate when they make him emotionless. The appeal is seeing that same fiery heart learning to burn more efficiently.
There's this one on AO3 where he returns and his magic feels ancient, like dragonfire mixed with something else. Lucy's trying to document his new abilities in her diary, and it's through her notes we see how scared she is of the gap between them. That kind of quiet, observational hurt/comfort hits different than the usual dramatic reunion scenes. It makes the eventual reunion, probably over a shared bowl of ramen, mean so much more.