He's just a place holder dude.
Roger, Rayleigh and Crocus never really believed Shanks was like Roger much. Roger even thought his son would become PK, potentially implying Shanks was only to give Ace the hat, which may have been why he was in the east blue in the first place.
Oda even used Shanks and Rayleigh interchangeably in SBS when talking about COC
They both dress similar, are similar in personalities, are viewed similarily by the world(Feared, but not hated or resented by regular people, unlike Roger, as you can see in Ace's childhood)