So the other day I was reading about Rokushiki, and Rokuogan got my eye. The seventh of the six mystical techniques, accessible to only those who had fully mastered all six. This got me thinking, What if this is also the case with Haki? What if there is a fourth form, the ultimate form, accessible only to masters of all three?
So let's say there was a fourth Haki form. What would it be? Well, let's look at the other Haki types. If you study the established three types, it is clear that each type has something in common. Basically, Haki is the act of forming a presence in nature, and making yourself a significant aspect in the world. Kenbunshoku Haki gives you the ability to sense the nature around you, Busoshoku Haki gives you the ability to shield yourself from nature, and even interact with it (as in hurting Logia users), and Haoshoku Haki gives you the ability to exert your presence on nature. So it can be determined that to master Haki is to master nature, the workings of the entire world.
So if Haki is all about gaining control of nature, then obviously the ultimate form of Haki would give you complete control. So what is it? Here's my prediction. Those who have mastered the fourth form of Haki will be able to manipulate nature as they see fit. They can cause living organisms to grow rapidly or die rapidly (though not resurrect them), and make objects move with the force of their minds, akin to telekinesis. Yes, I know this sounds incredibly like The Force, but if you think about it Kenbunshoku Haki is the same as the "I sense your presence" part of the force, so that argument is invalid.
Mastering a fourth Haki near the end of the series would be a good progression for Luffy, as the Gom Gom Fruit can only go so far, and especially toward the final battle. Luffy will almost certainly battle Akainu near the end of the series, and Sakazuki has shown that he's immune to Busoshoku Haki. So it would make sense that it would take a new power to overcome the Fleet Admiral.
Now, does anyone else have (or had) this Fourth Haki? Maybe. Here are my baseless prediction:
Gol D. Roger
In the Strong World chapter, we saw Roger overcome Shiki's entire fleet, which had his ship completely surrounded. Now he may have just gotten lucky, but if that many ships are surrounding you one is bound to meet its target eventually. My theory is that Roger used this Fourth Haki to use nature itself against Shiki's fleet. Not only that, but he had the power to talk to Sea Kings, and the ability to talk to animals without being a Zoan is unheard of so far in the series. Plus, this would go hand in hand with mastery of nature. A cardinal rule is that if you master something, you must have a special connection to it, which includes communication. Since Luffy also has this power, it may be a big sign that he is on his way to unlocking this. Also, he's the King of the Pirates, if anyone would have this power he would.
This brings me to another theory, which will be my last one. Let's say that I was right, and Roger manipulated water against Shiki's fleet. One of the main weaknesses of the Devil Fruits is the inability to move in water, but what if this weakness could be overcome? I mean, Logia are normally intangible, but this can be overcome by Busoshoku Haki. We obviously can't have a Water Logia, as that's a self-defeating power, and a DF allowing you to control water would be way too OP. As I've said before, this would be an ultimate form, and the ability to manipulate water and not be affected by it, in conjunction with your Devil Fruit powers, would make you nearly unstoppable. Before you complain that this would be too OP, this would be the ultimate result of the hardest training, not some Biju or Sharingan-crap like in Naruto. Plus, if One Piece is all about overcoming your weaknesses, what better weakness to overcome at the end of the series than one that has clearly existed the entire time?
This is my overtly long theory. Feel free to share your thoughts on this, or put your own theories in the comments.