Here is a guess about these two characters.
I find it suspicious how Shanks was implicated into the whole Ace and Teach fight. We know that Rayleigh knows about the void century, the will of D, he knows everything. It is the same for Shanks, problably, since he was in Roger’s crew as well.
Going with this, I believe that Shanks try to find “that man” that Crocus, and Whitebeard were talking about. Of course we know it’s Luffy, but Shanks as a character, I think his duty is to find that man, and he kinda did it with Luffy and the whole bet about the new generation.
Here is another example of Shanks ‘duty’ when he says to Cody that these few seconds of courage “have just made a huge difference in the fate of the world”, and these seconds of courage have indeed, distracted Kizaru’s shot that was going to finish Luffy. Shanks definitely sees luffy as the man that will change the world, the man awaited by Roger and co.
Now about the Ace vs Teach confrontation. Why did Shanks want to prevent this? “It’s not yet time for them to face each other!!” What is the meaning of that line? Is it the idea of two “D” confronting each other? It make me think about Luffy vs Teach. Two “D” confronting each other might inevitably lead to big changes in the world? That’s the case for Ace vs Teach, and it will probably be the case when Luffy will face Teach.
Now about the relationship between Shanks and Teach:
Why is he so concerned about Teach? Yes, there is the scar, but that’s it? I think it goes way beyond that. Now, theory time. I think that Shanks was friend with Teach.
I don’t think that Shanks could have deduces all of this, simply by having a fight against Teach. Shanks knows him pretty well, his ambition, his dream, his real power and so on. Why didn’t he say all of this to WB? First, because WB would not care and Shanks knows it, secondly, Oda might keep all of this for a flasback involving Shanks and Teach.
Now about the scar, the most important element:
“What ashes me now, is this one”. Really? Shanks lost his arm, suffered various wounds, he is an emperor, yet this simple scar would hurt him? I think the meaning behind this line, is that Shanks suffers from this scar because of the meaning behind it, not physically. If they were friends, I can totally imagine Teach betraying Shanks for some reasons.
But when would they meet? Why would they talk each other? Why would they become friends?
“This wound isn’t from my adventures”. I think it’s safe to say that by “adventures” Shanks means his journey with Roger, and his journey with his own crew.
Here is a timeline:
I think that longtime ago, after Roger’s death, Shanks met Teach and have seen in him the man that was awaited by Roger and co. Teach is a D, and Shanks could have been interested by him just like he was with Luffy. Shanks made a mistake and picked the wrong guy, that’s my guess. Why is Teach so confident about his fate? What does he know about the D? I think he knows almost everthing, thanx to Shanks. And that’s why Shanks might be so concerned by Teach. He was still young, and he made a mistake. He told him everything. Learning from his mistakes, he then did the opposite with Luffy, telling him nothing but just giving him hope and motives.
Why did they fight? Being a nobody (Teach’s childhood picture), I guess that knowing all of this story and his incredible fate, it changed him. He might have become greedy and power hungry from this moment.
They will probably end all of this with a fight.
So yea, that’s my 2cents about this.
*Theory by Kirk