I've stumbled upon some fantastic Harry Potter x Luna Lovegood fanfics over the years, and a few of them are quite lengthy! One that comes to mind is 'The Quiet Ones'—it's a slow burn that explores their dynamic post-war, with Harry drawn to Luna's eccentric wisdom and quiet strength. The fic dives deep into trauma recovery, magical theory, and Luna's unique perspective on the world. It's around 150k words, so definitely a commitment, but worth it for the character development.
Another epic is 'Luna's Year,' which rewrites the Deathly Hallows from Luna's perspective, with Harry as her unexpected anchor during captivity. The author nails Luna's voice—whimsical yet piercingly insightful. The romance builds naturally over 200k+ words, woven into a larger plot about healing and rediscovering magic. If you like world-building that feels true to Rowling's style but with darker, more mature themes, this one's a gem.