My idea (though it might look strange) would be to use graduated service lines on the court. Taller players would have to serve from lines further back that were made based on a mathematical equation that would equalize the heights by increasing the serving distance from the net.
The slope of the line that goes from the service line to the top of the center of the net is 1/7 or 1 foot rise for each 7 feet of run. The distance from a service line to the opposite baseline is 60 ft. The height of a ball over the baseline on that imaginary line with that slope would be about 8' 7" or 103 inches. The average pro player makes contact with the ball at 1.5 times their height according to studies made by Bruce Elliot.
So a player would have to be about 68 2/3 inches tall to hit a flat serve on a line that hit's the opponents service line*. A player 6 ft tall would have to stand 3 ft behind the baseline to serve a ball on that same flat slope trajectory as the player 5' 8.67" on the baseline. A player 6' 8" tall or 80" would need to serve about 10 ft behind the baseline to achieve the same slope.
Though it seems more fair, I doubt if many current taller players would agree to this change.
*Of course, this is without needing to account for gravity or ball speed or spin. Higher ball speed reduces the time gravity has to act on the ball. Lower speed and topspin will help get the ball in the box easier.
Respectfully,
masterclass