There's definitely the "King not being a swordman" concept that Oda definitely didn't need to introduce so late in One piece when until now he had been counting bizare people as swordmans if it didn't served any purpose in the story.
There's definitely more to this.
Plus times and again Oda has highlighted WB and Roger the strongest in One piece.
He also coincidentally didn't show whitebeard or roger really fighting. Whitebeard not even using his fruit from what we'd seen and roger probably holding back just as much
Maybe roger had a fruit or something that shanks eventually got his hands on. He was the one that took the gum gum fruit from cipher pol so maybe he was looking for it. Similar to how teach was looking for the yami fruit and then taking his captains fruit
If roger did end up having a fruit id rather shanks have it than it be the gum gum tbh