What an individual would have done is of course impossible to say, but there are some clues out there that might help make an estimate. Coltrane was part of a certain zeitgeist that was fueled by the turbulence of the civil rights movement, the anti-war movement, feminism, ecology, etc. Indeed, Coltrane was the perfect avatar for these turbulent events and his music a powerful representation of them.
With the end of the Vietnam war and the codification of key civil rights legislation, the steam went out of the American counter-culture. The free jazz movement dissipated. Its key leaders, Coltrane and Ayler were dead (almost symbolic deaths), and those that remained, even those close to Coltrane, like Pharoah Sanders, could not sustain "energy music" and moved towards less challenging or less confrontational musical expressions. The mid-70s saw the ride of the yuppie and the 80s the rise of Reaganism (I'll leave it there, not interested in getting into politics). That sort of put paid to what Coltrane was doing in the 60s. I don't think Coltrane could have bucked the trend of the times.
If one wants to see how it might have gone, one looks towards Europe: Peter Brotzmann and the German free jazz movement, and Derek Bailey and Evan Parker (among others) in England. They sort of picked up the ball and carried it forward. Their playing might show you the direction Coltrane might have taken, but I don't think he could have done that himself. Anyway, just some thoughts.