Let's see. Johnson authorized the Warren Commission to fabricate a 1200 page document that included forensic evidence, witness testimony, etc. All of those on the Commission therefore were
a) So cowed by Johnson's threats and pressure that they went along with it or
b)were involved in the plot themselves

How about this one. The mafia had the assassination done and Johnson ignored this because
a) he knew the mafia liked him and wouldn't subsequently do it to him
b) the mafia helped Johnson in his election and therefore he couldn't implicate them
c) his ambitions were so clearly opposite JFK's that he wanted him dead and he knew his mafia pals would be skilled at covering their tracks

Or this. Castro had it done, but Johnson wanted to pin it on Oswald because
a) He knew Castro liked him and wouldn't subsequently do it to him
b) Blaming a communist leader for the assassination might inflame the American populace against communism

Like the theories involving George W destroying the Trade Center in order to justify the invasion of Iraq, all of these scenarios are unlikely not just because the preponderance of evidence does not bear them out, but also because the depth of the involvement and dedication in carrying out and covering up these actions by a fairly large group of people would be nearly impossible to disguise.

I say "nearly", because I am quite sure that there are those who would buy the argument that the CIA has in its possession a time machine which was used to send John Wilkes Booth back to 1865 to kill Lincoln in order to prevent Abe from assuming the title of King of America, with which the leaders of the Army of the Potomac were planning to present him three days after the play. How else do you explain that Lincoln's wound was caused by a 357 Magnum round, a caliber that was not invented until 1916!