That's a fantastic question! Designing a lights-out factory PLC architecture from scratch today would be like building the ultimate automation playground. Here's what I'd go for:

For the core PLC platform, I'd choose a hybrid approach: Siemens for the main process control and Rockwell Automation for high-speed motion applications. Siemens' TIA Portal ecosystem is incredibly robust for complex automation, while Rockwell's ControlLogix platform excels at precision motion control. This gives you the best of both worlds - German engineering reliability and American motion expertise.

For communication protocols, I'd build everything around OPC UA over TSN (Time-Sensitive Networking) as the backbone. This emerging standard combines OPC UA's vendor-neutral data modeling with TSN's deterministic timing - perfect for lights-out operations where everything needs to work in perfect sync. For field-level communication, I'd use PROFINET for process areas and EtherNet/IP for motion-heavy zones, with both running on the same TSN infrastructure.

Why this combination? OPC UA TSN provides the future-proof, vendor-neutral communication that lights-out factories need for seamless integration between different systems. The Siemens-Rockwell hybrid approach gives you access to the strongest ecosystems in industrial automation, while the dual-protocol strategy ensures compatibility with existing equipment and future expansions.

I'd also layer in cybersecurity at every level - hardware security modules, encrypted communications, and redundant PLC architectures with automatic failover. After all, a lights-out factory needs to keep running even if something goes wrong!