Have you ever worked in a distributed system? If not, try one. If yes but you don't feel it hard, try improving its reliability/availability/scalability/performance/latency/debuggability/observability/consistency one order of magnitude (e.g. one more 9 in addition to 5 9s) at a time with one order of magnitude more distribution (e.g. spreading a global service to 10x more clusters/data centers).