Archive of Our Own consistently has the freshest 'Welcome to Night Vale' content in my browsing. The tag system means you can filter for exactly what Cecil/Carlos dynamic you want, and the writers there seem to really get the show's weird, poetic, and subtly ominous vibe. I found a series that imagines Carlos as a scientist trying to document the town's anomalies through field notes that read like lab reports mixed with love letters, which felt so perfectly in-character.
Tumblr is less of a formal platform and more of a wild ecosystem. The stories are buried in text posts, asks, and threads, but the interaction is immediate. You'll get snippets, headcanons, and drabbles that sometimes evolve into full narratives through reblogs and additions. It's messy, but it captures the communal, ever-expanding lore feeling of Night Vale itself.
Honestly, the Discord servers attached to bigger podcasts or fan blogs are where the real-time, hyper-specific discussions happen. People will just drop a three-sentence idea about StrexCorp taking over the local Ralphs, and it sparks a whole thread of micro-fics.