Robert E Lee- stays loyal to the union, crushes the southern army, and ends the rebellion in 1861. Later elected president top bridge the gap between north and south (or the cliche "independent south").
Aaron Burr- Alexander Hamilton's "stop Burr" coalition fails. Jefferson sinks into obscurity, Burr becomes recognized as a relativly good president for some reason (maybe something to do with setting up the "modern US", including strong central government).
King George III- A compromise deal is reached between the colonies and Britain proper, arbitrated by George III, which allows them to become an independent nation with the same soverign as the home country.
Lord Cornwallis- commemorative dollar bills, possibly in the George III timeline, or maybe celebrating his triumphant capture of Quebec (overdramatized version of Wolfe's attack).
Karl Marx- he emmigrates to America, which gives American socialism stronger roots. After a socialist revolution in the 1920s...
Superman- Either a commemorative bill for the comic or a politician who looks just like Clark Kent.
Benedict Arnold- Arnold and Montgomery take canada, and arnold takes credit. He gets some more rewards from congress, maybe a Saratoga type victory (probably near New York City or in the South-Savannah, say?) ensure his reputation as a hero. A short but distinguished political career, and voila.
Nancy Reagan- she goes into politics on her own, wins the presidency, and makes a name for herself as a social worker and in connection with various humaitarian groups.
George Custer- he has an ounce of the military skill he thought he had, so does better in the civil war, has a few successes out west, and transforms that into the presidency, then does something he will be remembered for.
Lafayette- easy enough. probably a commemoative bill, or maybe he stays in America and gets elected something.
or Sitting Bull- an america more accepting of Native Americans, but otherwise similar to OTL.