Swede Duplantis set a world record of 6.18m in February and last week surpassed Bubka's 1994 mark for the highest outdoor vault of all time.
"6.20? 6.25? For me, he is ready for that today," Bubka, who broke the world record more than 30 times in his own career, told BBC World Service Sport.
"He's an amazing athlete who's bringing a fresh spirit to pole vaulting."
Duplantis will bring his season to a close at the final Diamond League event of the year in Doha on Friday, competing in a field that includes American double world champion Sam Kendricks and Duplantis' predecessor as world record holder, France's Renaud Lavillenie.