What should be in a MERN MVP scope?

Three things only: the core user journey that proves the value proposition end-to-end, the smallest authentication and account model that works (email plus password, no social SSO yet), and the minimum admin dashboard your team needs to operate the product after launch. Everything else moves to a written post-MVP roadmap. We push back on scope additions during the MERN MVP build because the gold-plated MVP that ships in month nine is no longer an MVP.