I would say that they should build it as modular and clean as possible. With modular i mean possibility to rewrite parts of the system in different languages et

I would say that they should build it as modular and clean as possible. With modular i mean possibility to rewrite parts of the system in different languages etc. A distributed system with multiple copies of each module running on different machines in different locations. Build it around a "live" monitoring and control system. Use the base from the last 50 years, and add tested modern practices. This would include the possibility to upgrade any part of the system without taking it offline. From there, the sky is the limit.