I kind of what to see a trivially small web application built to scale as large as it possibly can.
Build tooling, deployment, productionization, monitoring, logging, alerting, unit testing, end-to-end testing, #webperf testing, #a11y testing, security, backups, replication, ...
It's incredible just how many concerns modern developers have and how many don't actually scale with application size and are things that even a small but mature application would eventually want to implement.