Natsu and Wendy fanfic that leans into their magical adventures? AO3 is pretty much the center for that kind of stuff. The 'Team Natsu' or 'Fairy Tail' tag has tons, but the trick is using the right filters. Tag 'Natsu Dragneel', 'Wendy Marvell', and then maybe 'Adventure' or 'Magic'. I'd also throw 'Sibling Bonding' or 'Found Family' into the mix, because that's the core of a lot of the good ones – it's less about romance and more about that big-brother, little-sister dynamic they've got going, which is perfect for quest narratives.
There's this one I read last year, can't remember the title, that had them tracking down a lost Sky Dragon artifact. The writer really got into the mechanics of Wendy's Sky Dragon Slayer magic in a way the manga sometimes glosses over, and paired it with Natsu's more brute-force Fire Dragon style. It felt like a proper side story from the guild's board. You'll find a lot of AUs too, where they're not in the guild but still have their powers, exploring some enchanted forest or ancient ruin together. The quality varies wildly, but sorting by kudos usually surfaces the gems.
Don't sleep on fanfiction.net either, even if it's older. The search is clunky, but there's a ton of material from back when the Tartaros arc was airing. You just have to wade through more abandoned fics and crossovers. I sometimes find the prose there is a bit more straightforward, less focused on tags and more on just telling a fun adventure. It’s a different vibe, less curated but with some surprising depth if you’re patient.