Is every store's catalogue loading, and can customers actually check out?
A store's catalogue can silently stop loading, or its checkout can start failing, and nothing tells you — you find out from a support ticket a day later.
A live status board across the fleet: which stores are pulling their full catalogue, which are stale, and whether checkout is reachable. It's the screen to open first when someone says “the website is broken”, because it tells you whether that's true everywhere or at one store.