Replay the game, partake in some of the other activities you may not have during the first playthrough (those that are still available), etc. If you're hung up on keeping your character totally "good", create a save and then start trying some of the activities that this game is all about (outlaws for life). .....try robbing stores, trains, people, etc. Try getting better at those activities so you're ready for online.
RDO is on the horizon so that will also have things you can do solo, coop or against other players in the game world. It's not as if the game dies after the campaign. .......unless you're completely against the online experience before we even have details of what it will consists of.
IMO, my friends and I can't wait to play together (co-op) when RDO arrives.