Kozuki Oden was a top tier level character. Someone who was ~ Yonkou level and the way Oda has continued to hype him post-flashback just re-emphasises this.
Due to his immense power ranking, some fans think Zoro surpassing Oden would be too much. This is probably because they've spent years underplaying Zoro's potential and some think he will barely reach Yonko Commander level if at all let alone Yonkou level lol.
Anyhow...
The biggest hint that Zoro is meant to surpass Oden is the fact that he has inherited Enma from Oden. Furthermore, Hitetsu specifically mentions how Enma hasn't yet turned into a black blade, and that by doing so its rank will increase. This is a deliberate benchmark added in for Zoro.
So why does turning a black blade signify Zoro surpassing Oden? Well let's look more closely at what Hitetsu said:
The key thing is Hitetsu says "Depending on you", that's the biggest giveaway that turning a sword into a black blade is dependent on the user's skill/power level. Otherwise, he could have said something like 'Its rank might yet be raised if it turns into a black blade.'
So the question arises why couldn't Oden turn his sword into a black blade? He was a top tier after all. That's because turning a sword into a black blade is no small feat, it's probably the pinnacle of CoA swordsmanship, hence why the only known person to do it is Ryuuma who's worshipped by Wano people. Mihawk could have too, and he just so happens to be the World's Greatest Swordsmen.
Some of you may now say well then what about Roger/Shanks, they didn't turn their swords in black blades either...
The answer to that is, the parallels between Oden and Zoro are more apparent compared with the other two. Just in terms of fighting style, Oden's two sword style is practically the same as Zoro's two sword style. Oden's haki sword attacks are basically the same as Zoro's and so on. So Oden is much more of a benchmark for Zoro to eclipse, and Zoro may indeed surpass Shanks, if Shanks = Mihawk.
Due to his immense power ranking, some fans think Zoro surpassing Oden would be too much. This is probably because they've spent years underplaying Zoro's potential and some think he will barely reach Yonko Commander level if at all let alone Yonkou level lol.
Anyhow...
The biggest hint that Zoro is meant to surpass Oden is the fact that he has inherited Enma from Oden. Furthermore, Hitetsu specifically mentions how Enma hasn't yet turned into a black blade, and that by doing so its rank will increase. This is a deliberate benchmark added in for Zoro.
So why does turning a black blade signify Zoro surpassing Oden? Well let's look more closely at what Hitetsu said:
The key thing is Hitetsu says "Depending on you", that's the biggest giveaway that turning a sword into a black blade is dependent on the user's skill/power level. Otherwise, he could have said something like 'Its rank might yet be raised if it turns into a black blade.'
So the question arises why couldn't Oden turn his sword into a black blade? He was a top tier after all. That's because turning a sword into a black blade is no small feat, it's probably the pinnacle of CoA swordsmanship, hence why the only known person to do it is Ryuuma who's worshipped by Wano people. Mihawk could have too, and he just so happens to be the World's Greatest Swordsmen.
Some of you may now say well then what about Roger/Shanks, they didn't turn their swords in black blades either...
The answer to that is, the parallels between Oden and Zoro are more apparent compared with the other two. Just in terms of fighting style, Oden's two sword style is practically the same as Zoro's two sword style. Oden's haki sword attacks are basically the same as Zoro's and so on. So Oden is much more of a benchmark for Zoro to eclipse, and Zoro may indeed surpass Shanks, if Shanks = Mihawk.