Honestly, a prequel set during Dutch's gang was the laziest possible choice Rockstar could have made. Also see a LOT of familiar stuff in that trailer.
I really hope this wasn't pushed out just to get RDR Online 2. It can't be anywhere near the scale of GTA but I'm sure they'll replace Shark cards with "bank notes" and make a ton of money all over again.
Having said that, Dutch was my favorite character from RDR. He was only in 3 scenes but the guy was magnetic. If the gameplay is on par with GTA 5, I'm sold. If the game actually let's you play as an outlaw, even better.