[Manga Spoilers] Shanks may have mark/symbol.
I am not sure if this theory has been already made.
When we were at Marineford, we always wondered. HOW DID SHANKS QUICKLY CAME TO MARINEFORD From new world where he was fighting Kaido. Even marines thought the same.
Now that we know, If you have some mark which are present on Holy Knight and gorosei, you can easily transport with some condition. Location should have mark/symbol, so anyone can transport there. Another is a person having mark can call another person having mark.
What this condition are we still don't know. Maybe when Shanks visited Mary Geoise, He either got the symbol or he learned the trick of how to do it. That's how he was able to travel to Marineford that quickly. Maybe he sent a scout ahead already. With help of that he was able to quickly travel.