Last September, two months before his arrest in Tokyo, then-Nissan Motor Co. Chairman Carlos Ghosn and the carmaker's chief executive officer considered bringing in a new partner for the alliance with Renault SA and Mitsubishi Motors Corp., according to an email.

At the time, Ghosn was under pressure to make the three-way automobile alliance "irreversible." In a message to Ghosn seen by Bloomberg, Nissan CEO Hiroto Saikawa wrote that he had been working over the summer "quietly by myself," at his boss's request, to find a structure that would be "acceptable for both sides." He offered to discuss possibilities with Ghosn.

In the correspondence, Saikawa raised the possibility of bringing in another manufacturer as a fourth partner for the alliance. He didn't identify any potential candidate. He wrote that expansion opportunities also included "acquisition of Chinese companies" for electric vehicles or connected services.