Shard-key choice: sharding users by user_id is smooth, but tweets by user_id hurts with a celebrity account; sharding a log system by date creates a hot current-day shard.
A second full mock with you leading end to end (URL shortener or a news feed), using the question checklist.
Redo the estimation math out loud on a bigger problem (a Twitter-style feed).
Done since the first version: the notification system mock, protocols and GraphQL, auth (sessions, JWT, OAuth, API keys), authorization (RBAC/ABAC, where auth lives, IDOR), microservices (boundaries, sync vs async, database per service, failure isolation), observability (logs, metrics, traces, alerting, SLI/SLO/SLA), sagas and the outbox pattern, consensus and leader election, backpressure and load shedding, stream processing, disaster recovery, the senior-level signals, end-to-end encryption, ordering and acks, and the chat system mock.
Summary: say it in the interview
Habit to build: after the boxes, always add "here is where it breaks at 10x, and what I would change".