Every app idea has a boring 90% and a nasty 10%. The boring part is CRUD, auth, a settings page, Stripe. The nasty part is the one subsystem that decides whether the thing works at all, and it’s usually not the part that looks hard in the pitch. Below are nine builds worth attempting, with the nasty 10% named up front so you can decide whether you want to spend your weekends there.

The default architecture for most mobile applications treats network connectivity as a reliable precondition. API calls are made on demand. Failures produce error states. The user waits for responses. This architecture produces apps that work acceptably on fast, reliable connections and badly on slow, intermittent, or absent connections — which describes the conditions under which many mobile users actually use apps.

Offline-first architecture inverts this assumption. The app reads from and writes to local storage first. Network synchronization happens in the background, opportunistically, whenever connectivity permits. The user experience is fast and available regardless of network state. The complexity is in the synchronization layer, which most teams underestimate significantly before they build it.

Every mobile app that does anything interesting eventually needs a backend. Authentication, data storage, push notifications, file uploads, real-time updates — the list of backend requirements grows quickly once an app moves beyond a local-only experience. The decision of how to provide that backend is one of the most consequential architectural choices a mobile development team makes, and it is one that becomes substantially harder to reverse after significant user data has accumulated.