When Masayoshi Son made a surprise appearance at Mar-a-Lago alongside Donald Trump last week, the founder of Japan’s SoftBank Group Corp. vowed to invest a stunning $100 billion in the US over the next four years. The president-elect joked that perhaps SoftBank would like to double that target to $200 billion. Not to be outdone, Son replied with a wide smile, “I will really try.” That may not be an exaggeration. The Japanese entrepreneur is quietly plotting one of his trademark bet-the-company m...