If Johnson could have been convinced to fight a boxer of color, this might have been a damned interesting bout.  Johnson, not as mobile as Ali, tries to trap him on the ropes without success for most of the fight, taking a fair bit of punishment while doing so.  Late in the fight, knowing he's behind on the cards, he takes it to Ali, trying to trade punches with the lighter and faster man.  It doesn't work.  Ali withstands the barrage, gives as good as he gets, and wins a close decision.

Unless Jack Johnson hit him with a bomb (think Ken Norton), Ali would have danced, hit, and taunted Johnson into submission.

Hmmm? Interesting fight in that both were known for their brash attitudes, resilience, ring generalship and their relentless showmanship. But when taking skill into account, Jack Johnson was a relatively stationary fighter, especially when compared to the constantly moving and flowing Muhammad Ali, and thus would have created problems for the Galveston Giant. Jack Johnson was very similar to an older, wiser George Foreman who would fight defensively until he found an opening to exploit his opponent's weakness- and then would unleash his power. Yet, in facing Ali, Johnson would have had to account for the fact that he would likely be losing on points late during the fight (on which he would have found little weakness in the Great One), and would likely thereafter take risky chances to try to change the momentum of the bout. But no knockouts. Ali would never have had the power to knockout Johnson, and would likely have had a firm decision against him instead.

Jack Johnson vs Muhammed Ali