Since the first few chapters where we saw Shanks for the first time, it was clear that he'd be one of the strongest characters in the manga.
That's just how it is.
I always thought he (and whitebeard) will be shichibukai too.
But being one of the four emperors is even better since they are not lackeys of the WG.
In the end when Ruffy and Shanks will meet again everything could happen
"You've become a great pirate, Ruffy. Now let's fight"
"You've become a great pirate, Ruffy. Let the hugging begin"
Both will be good, I guess *crosses his fingers for fighting*
I just hope Ruffy will keep the strawhat. He needs it.
"For Feanor was made the mightiest in all parts of body and mind, in valour, in endurance, in beauty, in understanding, in skill, in strength and subtlety alike, of all the Children of Illuvatar, and a bright flame was in him"