Shanks and Teach have, as of volume 96, appeared on five volume covers each. They also happen to be the same five volumes: 25, 45, 59, 95, and 96.
So Shanks and Teach have always appeared together on the five volume covers they’re each featured in. One has never appeared on a cover without the other, and I doubt that’s a coincidence. I think this helps to highlight the importance of their connection.
They also appear together in the third and final One Piece Global Character Popularity Poll Top 50 Illustration by Oda.
Oda’s definitely been foreshadowing that they will fight eventually, especially with their little clash at Marineford.
And I’d also not be surprised to discover they have some connection through God Valley and Xebec:
- The God Valley incident occurred 38 years ago. Shanks is 39 and Teach is 40.
- When Oden and Toki join the Roger Pirates, bringing Momonosuke and Hiyori with them, Roger and Rayleigh both hint that there had been a baby or babies with the crew before.
- There were Celestial Dragons and their slaves on God Valley during the incident. Shanks seems to be able to get an audience with the Five Elders with no trouble. Maybe Shanks was a Celestial Dragon that the Roger Pirates found/rescued/kidnapped as a baby during the incident. Maybe Xebec killed his parents.
- The Blackbeard Pirates’ flagship is named “Saber of Xebec”.
- Teach, a D. just like Xebec (or so he claims), seems to be carrying on his goal of becoming King of the World. He’s also an orphan. Maybe Xebec was his father. Even if they’re not blood-related, there has to be some connection.
*Theory by SgtPepper212