What would a Bill Gates comeback look like?

It wouldn’t be the first time a legendary founder stepped back into the fray to right his faltering company. Perhaps Gates has noticed the success of his Seattle neighbor Howard Schultz, who returned to a badly bruised Starbucks (SBUX) only to help it surge. (Schultz was Fortune‘s 2011 Businessperson of the Year.) Michael Dell, Steve Jobs and Larry Page all pulled similar maneuvers.

It wouldn’t be the first time investors had flirted with the notion, either. Early this year, activist investor David Einhorn of Greenlight Capital publicly called current CEO Steve Ballmer a weight dragging down the company’s share price, prompting the rumor that Gates might return. But Gates has been cool to the idea publicly, rebuffing it in 2010 and again this year in June, when he told the Daily Mail his work at the Bill & Melinda Gates Foundation “is my job now.” (Einhorn declined to comment for this story.)

One prominent chief executive told Fortune he’d heard from someone close to Gates that he might be considering such a move. Aside from an unexpected and bold final act for Gates, what could Microsoft (MSFT) gain from such a move?

The company’s stock price has been a thorn in Ballmer’s side for a decade. At the end of 1999, Microsoft’s best year for market price, shares were at almost $60. When Ballmer took over as chief executive in January 2000, the stock was still over $50. With the burst of the tech bubble, Microsoft’s stock dropped steeply and hovered around $25 area through much of the decade, excepting a short rally above $35 in late-2007 and a dip under $20 during the 2008 financial crisis. Ballmer dumped much of his stock during the decade and Bill Gates gradually left daily operations. (Gates’ last full day was in June 2008.) During that time, the stock stayed flat as competitors such as Oracle (ORCL) and Apple (AAPL) saw huge gains in their share price.

Could Gates goose the moribund stock price? Possibly. Any CEO turnover tends to jumpstart improvement in a company’s stock price, says Jarrad Harford, finance professor and chair at the University of Washington’s Foster School of Business in Seattle. According to Harford, investors anticipate the company’s value under new management even before the executive actually takes over. (New CEOs have about a year before his or her imprint needs to be visible to investors.) “There’s a lot of frustration among investors with Steve Ballmer,” adds Ed Maguire of CLSA Asia-Pacific Markets. “Some is merited, some may not be. But there’s certainly a perception that Ballmer is responsible for the under-performance of the stock.”

Then there’s the effect a founder can have on the troops. Founders are generally free to execute their vision with added “gravitas,” Harford argues. He points to Google (GOOG), where co-founder and again-CEO Larry Page is “cutting through the organizational muck” to restore the company’s momentum. Ballmer might lack that incisive big-picture vision, says Maguire at CLSA. “You don’t have the benevolent dictator,” he notes, adding that while Ballmer handles the active management of myriad divisions and thousands of employees well, he’s missing some of the messaging of a Jobs or Gates.

Gates’ return could help streamline Microsoft’s message, but Harford notes that even Steve Jobs at Apple (AAPL) had current CEO Tim Cook at his side to handle the nitty-gritty. It’s unclear whether Gates could keep his old right-hand man, Ballmer, or find a replacement. And, it must be noted that, during his tenure, Gates articulated more than one vision that went nowhere.

Analysts say that whoever is in charge at Microsoft will face serious headwinds — even Gates. Under Ballmer, the company picked itself up after the disastrous Windows Vista, launching Windows 7 successfully. Now, it is preparing Windows 8, which the company hopes will launch it into tablets. But it must draw developers to its platforms and duke it out with with Amazon (AMZN), Apple and Google. “There are very few examples of where returning founders had to come from behind in their certain market,” warns Kirk Materne of Evercore Partners, which carries out investment banking services for Microsoft. “Gates was great at Microsoft when Microsoft was the dominant player.” Even Gates, Materne warns, would face challenges.

Analysts and Harford agree that Ballmer’s position at Microsoft is well established, and he remains a close friend of Gates. It’s most probably, they say, that Ballmer stays in charge as the company brings its new products to bear in 2012 and beyond, at least until Ballmer turns 60 in 2016. A Microsoft spokesman noted the company does not have a fixed retirement age and that the company does have a succession plan in place but would not provide details. (He declined to comment on a Gates return to the company.) Insider Steven Sinofsky, who heads up Windows, might be the company’s best bet for a high-profile scion — “he even wears the black turtlenecks,” Maguire notes. Harford says that whenever the succession is, it will differ greatly from when Gates unilaterally passed control to his buddy Ballmer, as the board will have a much greater say.

Even if unlikely, Gate’s return is a “what if?” that’s likely to have life, especially as long as the company’s stock price languishes. But if he did return? “I wouldn’t hasten to bet against Bill Gates,” Materne says. He’s probably not the only one.