Jane Austen
Samuel Clemens, Ralph Bunche, Francis Bacon, Alexander Pope (hell, all the member of the Scriblerus Club), Ralph Emerson, Langston Hughes and Franz Boas.
Leonardo DaVinci would feel right at home here.
Ben Franklin would be popular.
Karl Marx would own Great Debates.
I wish John Glenn would sign up.
Or Burt Rutan.
I would love to see Eve and H. L. Mencken go toe-to-toe in a snark-off…
Anyone from the Alogonquin Table would be welcomed. Image Eve and Dorothy Parker in a snark-off.
I nominate Ben Franklin, for his wide-ranging knowledge of science and politics, but equally for his raunchy sense of humor. 
Twain! Oh my dear Samuel Clemens, is there no way to ressurect you?!?! Imagine all the TMI he could contribute to MPSIMS! 
Ben Franklin for sure, but what about Harry Truman and George C Marshall. Both seemed to have an ability to see the reality in events, looking past the surface.
On the other hand, both we pretty close-mouthed.
Wittgenstein would be fun until he got banned.
Daniel Webster
Hammurabi
Confucius
O. Henry
Not exactly a historic figure – but what the hell, it was my OP, if I can’t go off on a tangent, who can?
For sheer raw snarkery, I’d love to see Paul Lynde post in either Care Society or MPSIMS.
Kruschev would add some fire to the Pit.
And Harlan Ellison, until he was banned.
I would have liked to have a chat with Will Rogers sometime.
Isaac Asimov
Ronald Reagan,
Winston Churchill,
John Stuart Mill,
The Duke of Wellington,
Pepys,
Aristotle,
Plato.
Nietzsche
Hm. (An excellent thread topic/question.) Ha! Ambrose Bierce. What a delightful horror show that would’ve been. 
Thomas Jefferson. At least he could finally end the various SOCAS threads that are running at any given moment.
Most of the Founding Fathers. Jefferson and Franklin have already been mentioned, but Madison, Adams (John and Sam) Hamilton, Patrick Henry, Tom Paine, George Mason, John Jay…oh yeah, and George Washington. He’s got a rep as an intellectual lightweight, but look at the competition. He wouldn’t have posted a lot, but I think he could have held his own here when he did.
Oscar Wilde