I want to build something a reviewer hasn't seen a thousand times, but everything I come up with is either trivial or a six-month monster. Ideally something that shows real backend thinking — auth, data modeling, a bit of concurrency — without needing a design team to make it presentable. What did you build that got a genuine reaction in an interview?
Build something you'd actually use, even if it's tiny. A tool that scratches your own itch reads completely differently in an interview than a tutorial clone, because you can talk about the decisions with real conviction. Bonus if it has one genuinely hard part (auth, real-time, a tricky data model) you can go deep on.
A URL shortener sounds boring but it's secretly perfect for interviews, you get to talk hashing, collisions, rate limiting, caching, database choice for as long as they'll let you. Tiny to build, huge surface area to discuss.
Build something you'd use yourself. I made a little tool that pulls my utility bills and charts the usage, and interviewers loved that it scratched a real itch. The auth and concurrency stuff shows up naturally when the problem is real.
