Since the very beginning,it is forecasted that a huge war will occur between Straw Hats & Blackbeard Pirates.Mostly speculated that it will be in Raftel or just before Raftel.The outcome of the battle will decide the Pirate King and most probably it will be Monkey D Luffy.
I think I am heading out of the topic,let's get to the topic now which is the main theme of this part of theory ,please have a look at the image below:

Zoro VS Daz Bones(Alabasta Arc)

I think this is the ultimate forecasting created by Eiichiro Oda from the very beginning when Zoro cuts steel and Daz Bones replies after the defeat if he is going to cut diamond next.It surely implies that Zoro will be able to cut diamond in future.Yes,he surely will cut the diamond which has the highest hardness & thermal conductivity of any bulk material.As Zoro will become the strongest swordsman in future,he must be capable of it.

Mihawk vs Jozu:Let's take a look at the image below from Marineford Arc.

Mihawk vs Jozu(Marineford Arc)

Here we can see Mihawk wants to measure the power level between him and the world's strongest man "Edward Newgate" aka "Whitebeard" and sends his powerful slash of the sword towards Whitebeard but is stopped easily by Jozu deflecting the slash upwards with his bare hands.Can you imagine diamond deflecting the world's strongest sword slash attack so easily? What does that mean? Is it not possible to cut diamond through sword atttack? Is it that Dracule Mihawk titled with the world's strongest swordsman cannot cut diamond? I don't think so though I think he can but at least here we get to know that it's very hard to cut diamond.So,if the Roronoa Zoro is able to cut diamond then only he will be considered as a capable fighter on a par level with Mihawk to battle him.Maybe after that he could be considered one of the vary rare characters or only a character to cut diamond.

Now,since Zoro will cut diamond in future,considering the current situations he must battle with Diamond Jozu to do so.

Diamond Jozu turning into diamond

Whitebeard Pirates and Luffy are in a good relationship saving Luffy in Marineford considering him as the Ace's brother so to say they are actually in a very good relationship even we may see them allying Straw Hat Pirates in Wano Kingdom.So it's not getting anywhere that a dispute will arise among them leading Roronoa Zoro fighting Diamond Jozu. So,how will this happen,how will Zoro without battling Diamond Jozu be able to cut diamond?

Thankyou!! Will explain about it next part.Do follow me and feel free to comment down your views about this.