React fics usually follow a set pattern where characters get shown clips or articles from their future or from our world, and then they, well, react. The foundational trope has to be 'Canon Characters Watch the Show/Movie.' It's straightforward but lets you explore how they'd process their own story, which is the core appeal.
Beyond that, 'Reaction to Fandom' is huge—characters discovering fan theories, memes, or even fanfiction about themselves. That meta layer adds comedy and a bit of existential horror for them. 'Alternate Timeline Reactions' is another common one, where they see a 'what if' scenario play out, often to highlight how much worse things could have gone.
I find the most satisfying ones use the react framework to force unresolved emotional conversations. Like, watching a future argument might prompt the characters to hash it out right then, changing the present timeline. Without that character development, it just feels like a passive viewing party.