You know, if you're still hunting for Eren/Annie stuff years later, you've probably already scoured the usual big names like Ao3 and FF.net. The real trick isn't just finding the platform, it's finding the authors who stuck with that pairing post-canon when the fandom momentum shifted hard elsewhere. Most of the truly dedicated character work I've seen popped up on Tumblr blogs first, where writers would post drabbles and headcanons, and then sometimes cross-post longer pieces to Ao3. The tagging system over there is your lifeline – searching 'Eren/Annie', 'Annie/Eren', 'Eren & Annie', even 'Annie Leonhart-Centric' can pull up different subsets. A lot of the stories that dig into the complexities, the shared trauma of being weapons, the post-rumbling awkwardness... they tend to have more layered tags that hint at the themes. You'll find some surprisingly introspective stuff if you filter for 'Canon Divergence' or 'Post-Canon' and sort by kudos, but be ready to sift. The peak volume was definitely around the Female Titan arc and then again after her crystal breakout, so don't ignore fics from 2014 or 2018-2020.
Honestly, the quality is incredibly scattered. You get these brilliant short pieces that nail their dynamic in under 2k words – the silence, the understanding laced with resentment – and then a hundred forgettable high school AUs. I gravitate toward the ones that treat their connection as profoundly messed up but inevitable, not fluffy. For that, Ao3 is still the best aggregator. I'd almost recommend finding one author you like and checking their bookmarks; that's how I found a few hidden gems that never got big traction. Also, a weirdly specific niche: some of the best philosophical takes on them were on Quotev, of all places, but that site's a ghost town now and navigating it is a nightmare.