(FORTUNE Magazine) – The heroes of this bull market boom won't be around forever. Someday Bill Gates will leave Microsoft. Warren Buffett will retire. And one day--this one hurts the most--Alan Greenspan will turn off his PC at the Fed for the last time.

But don't panic, at least not about Greenspan: There are interesting replacement possibilities out there. Take a look at Roger Ferguson, a two-year member of the Fed Board of Governors just promoted to vice chairman, which means he'd at least temporarily take over if something happened to Greenspan. FORTUNE's betting he'll soon be near the top of the list of candidates to succeed Greenspan. (By most accounts the list includes former Treasury Secretary Robert Rubin, his successor, Larry Summers, and New York Fed president William McDonough.)

Ferguson may be the first vice chairman under Greenspan to have real influence. "He's a dream come true for the Fed," says Janet Yellen, former head of the Council of Economic Advisers and now a professor at the University of California-Berkeley's business school. (Yellen had a hand in Ferguson's appointment to the board, as did his Harvard classmate Larry Summers.) Yellen sees in Ferguson a rare combination of skills: a trained economist and lawyer with expertise in banking, management, and technology. At 47, his resume has the hallmarks of success: BA, law degree, and Ph.D. from Harvard; a position with the big-shot law firm Davis Polk & Wardell in New York; and a directorship at McKinsey & Co.

It's a background he has crafted carefully. This is a man who's wanted to be on the Fed Board for a long time--in fact, since he was a teenager growing up in Washington, D.C. What drives him, says the earnest Ferguson, is a deep fascination with the banking system.

If this makes him sound pompous, he's not. When FORTUNE paid a visit to him recently, he was happy to give a tour of the Fed boardroom and explain the nuances of its seating arrangements, but he was uncomfortable talking about himself. "Roger's not the sort to thrust himself in the middle of things," says his former dissertation adviser, Harvard professor Richard Caves. "That's what makes his success all the more pleasing." And unusual. Because Ferguson doesn't make a big noise, he can surprise people. No one expected his appointment to the board, much less his being singled out as Greenspan's deputy.

Ferguson is only now getting noticed by Wall Street. As one Wall Street economist told FORTUNE on condition of anonymity, "The more you talk to Ferguson, the more impressed you are by him." What seems to impress Wall Street is his monetary leanings, which economists characterize as neutral and sensible. Ferguson is sympathetic to the view, for instance, that we have a New Economy--lots of paradigm-shifting infotech, virtual corporations, knowledge workers, etc.--but would hesitate to have the Fed react until there is greater proof. "He doesn't have extreme views," says another Wall Street economist and member of the Fed's informal inner circle. "He's very much in keeping with Greenspan."

Whether Ferguson succeeds Greenspan comes down to politics. Ferguson was appointed to the Fed by Clinton and is seen by Democrats as their man, but he's probably enough of a centrist to appeal to Republicans. FORTUNE's prediction: Don't underestimate Ferguson. Even if he never makes it into the top job at the Fed, he is sure to be a key player for a long time to come.

--Anna Bernasek