After finishing the story in RDR2 Online, you could go for collecting rare items, engaging in PvP battles, or even start a new character to experience different playstyles.
You could explore the open world, take on side quests, or try different multiplayer game modes.
Well, you might want to focus on improving your character's skills and stats. Another option is to team up with friends and form a posse for more challenging activities. You could also just enjoy the scenery and discover hidden locations.