As suspected, Red Dead Redemption 2 is a prequel.
According to the description on today's trailer, Red Dead Redemption 2 is “the story of outlaw Arthur Morgan and the Van der Linde gang as they rob, fight and steal their way across the vast and rugged heart of America in order to survive”.
Dutch Van der Linde was the leader of the gang that Red Dead Redemption protagonist John Marston was a part of. However, Van der Linde lost his mind and became one of the primary antagonists of Red Dead Redemption.
Dutch Van der Linde is in today's trailer, and there's speculation that John Marston himself is too (at around 1:14, in the burning building shot), alongside his wife Abgail (at 1:03).
RDR2 protagonist Arthur Morgan is not someone we've come across before.
Red Dead Redemption 2 is slated for an autumn 21018 release on PlayStation 4 and Xbox One.