Finding stories where Natsu and Erza click always brings out different dynamics in the guild. I've spent way too much time digging through that tag on FanFiction.net and AO3, and the real popular ones aren't just one type.
There's a bunch where they accidentally get hit by some kind of love potion or magic, forcing them to confront feelings they've been ignoring. It's a classic, but some authors do it really well by focusing on Erza's internal conflict about duty versus personal desire, and Natsu's total confusion when his straightforward nature runs into something complicated.
Then you've got the 'hurt/comfort' angle that's huge, especially post-Tartaros or Alvarez. A fic where Erza gets severely injured and Natsu is the one who just... doesn't leave, and his simple, stubborn care breaks through her need to be the strong one. Those stories often have a great slow shift from comrades to something more intimate without big declarations.
A less common but fascinating one I keep seeing is AUs where they aren't in the guild—maybe they're rivals from different guilds, or a modern AU where they're college students. The push-pull of their personalities stays intact even without the Fairy Tail context, and watching them find common ground from scratch is its own kind of magic.