The theories are found here: http://orojackson.com/threads/the-death-of-zoro-chapter-944-odas-foreshadowing.9086/
And they came to life thanks to Oda's latest color spread 784 and to be exact, this picture:
I want to agree with Vandenreich's theory the most, as I see it impossible for Zoro to die so soon. If you have read these theories already, there is another one about Zoro's cursed sword:
This one is really really important and interesting, ok? Throughout the series, I have noticed Zoro to be somehow colder and after 2 years, this has surfaced even more. I don't say he doesn't care about his crewmembers anymore - he still does! It's just that the Zoro we first met isn't the Zoro now. He certainly has to change after all the adventuring but...you know, I was really just telling myself this but after I read more about the curse of Zoro's sword, I realized there is also THAT. His sword could also be the reason his personality changed a bit and I failed to notice this.
From the theory about Zoro's sword, I am actually surprised the curse of it wasn't mentioned anymore already (after it's first appearance). We will probably get to see more about swords and swordsmanship from the Wano country, which I believe Straw Hats WILL stumble upon. Probably after Zou, Dressrosa and what not. We will have to wait until then then...3 years huh.... anyway, let me know what you think about those theories?