Uh yes there sure is. You could have said that about RDR 1 back in the day. Because before RDR 2 came out we didn’t know Arthur or most of the other main characters. We didn’t know about the Pinkertons, the ODriscolls, the Braithwaites and Greys. All of those stories and characters are completely knew to the game universe.
Even if they didn’t come up with knew stories for an RDR 3 we still have a lot of stuff that happened in the past that we haven’t seen yet that they can touch on. For example Blackwater Massacre, John Marstons daughter who passed away, the Callender brothers, the war and history between Dutch and Colm (featuring Annabelle and Colm’s brothers maybe), Jenny Kirk, how Dutch and Micah met, Heidi McCourt. The Strange Man, Etc.