As executive director, Brown worked hard to make the FSF more of an activist organization, encouraging public information campaigns, and trying to establish alliances with academics, activists, and anyone else who might support free software issues.
Even the Open Source Initiative, which you might assume would be at odds with the FSF, seems to have appreciated his efforts at outreach. His success was limited, although whether a lack of funds or internal opposition was the reason is uncertain -- perhaps both.
Brown has always expressed orthodox free software positions, but his emphasis would probably be more on political action. One thing he said that has always stuck with me is that free software should be as much of an issue in the public mind as recycling.
More outgoing than Stallman, he would probably provoke less personal criticism, although many might object to his activist tendencies.
2) Benjamin Mako Hill has been active in Debian, Ubuntu, and One Laptop Per Child. He became an FSF board member in 2007 at the age of twenty-seven as part of Brown's deliberate effort to recruit younger supporters. He is a Senior Senior Researcher at the MIT Sloan School of Management, studying free software communities and business models.
Hill is a fluent although slow writer, whose ideas reflect the breadth of his experience in FOSS, and are more thoughtful than dogmatic. He would probably respond to new technology and circumstances with some flexibility -- even with originality.
However, since he used to describe himself as "a rebel with too many causes," Hill might spread himself too thin to lead successfully. Moreover, for all his experience, he may also be too independent to want a leadership position, or to use it effectively.
3) Bradley Kuhn: was FSF executive director from 2001 to 2005. He has been an employee at the Software Freedom Law Center, and is now executive director of the Software Freedom Conservancy.
Through Kuhn's blog, he has become increasingly quoted when free software issues make the news -- recently, for example, his views on Project Harmony, Mark Shuttleworth's group for developing templates for copyright assignment, have been widely repeated.
As FSF executive director, Kuhn was relatively quiet. Although several of the Foundation's long-time employees praise his work, externally, he seemed very much in Stallman's shadow. This appearance may be due partly to the fact that he was twenty-seven when appointed.
However, his recent comments on issues suggest that part of the reason may have been that his opinions differ little from Stallman's. My impression is that the FSF under Kuhn would act and look much the same as it does today. It might even revert to being an organization more for developers than for average users.
I have no idea if any of Brown, Hill, or Kuhn have any interest in setting the intellectual standards for free software. Since the question is theoretical for now, I saw no point in asking. I mention them mainly to suggest the range of alternate futures that might be in store for the FSF.
I am leaving open, as well, the question of how formal the succession would be -- whether it would be signalled by someone eventually replacing Stallman as president, or simply be a matter of informal influence. I could even see policy being set, not by an individual, but by votes at the LibrePlanet conference each year.
Still, the matter is worth some discussion, no matter how it is defined. Planning for what comes next is not merely a way of placating those who criticize Stallman's leadership and who suggest that he has outserved his time.
Rather, thinking about the future now will not only to help to decide what the FSF will be like in the future but also -- just possibly -- how much of a future the FSF has.