"Have any metro areas built entirely new subways where there were none before in recent times?" China is the easiest one to study. The number of kilometers of heavy rail metro lines built since the year 2000 is simply mindboggling. (Specifically, I am not talking about high-speed train lines.) In the same two decade time range, look at Korea, Taiwan, Hongkong, and Singapore. I guess multiple systems have doubled or tripled in size (length or stations). Remember that Korea has two major systems: Seoul and Busan; and Taiwan has Taipei and Kaosiung.
I am not an urban planner but the north-south & east-west initial two-line style seems very popular. In my experience, the trick to quickly increasing ridership on new lines is closely coordinate with private developers. Be transparent about line planning, then team-up with private developers who can build (large) residential or office buildings with subway entrances in their basement. I've never seen a city do it better than Tokyo. My assumption is that the Elizabeth Line in London will spur similarly spectacular levels of redevelopment.
Light rail: Silicon Valley opened their system in Dec 1987.
About "unlocking value": Two massive upgrades come to mind. East Side Access in Manhattan will allow LIRR trains to enter Grand Central Station. The construction photos online are like something from science fiction. And look at last 30 years of Tokyo metro tightly integrating with private suburban rail lines where trains enter same station, but opposite sides of same platform. Everything is timed to the minute, so transfers are seamless.