In this case, doesn't the urge to sequelize originate with MT himself?

Huckleberry_Finn is, obviously, a continuation of the characters and
settings which first appear in Tom_Sawyer.

Additionally, Tom_Sawyer_Abroad and Tom_Sawyer_Detective were completed
and brought to publication.

"Tom and Huck Among the Indians" and "Tom Sawyer's Conspiracy" and
"Schoolhouse Hill" were begun, but did not see completion.

Are there not also some MT rough notes about Tom and Huck returning to
St Petersburg as old men of 60 or so?

See also:
http://en.wikipedia.org/wiki/List_of_characters_in_the_Tom_Sawyer_series


DDD