The New Jersey Devils have two of the greatest "what ifs" in history, where they were very close to having both Wayne Gretzky and Mario Lemieux on the roster. They lost Gretzky because of the NHL rules for incoming players.
The year was 1979. The WHA agreed to fold as a league and was bought out by the NHL. Four expansion teams were allowed to hold onto their franchise and join the NHL: Edmonton Oilers, New England/Hartford Whalers, Winnipeg Jets, and Quebec Nordiques.