after john dies

There is only one mission left and it will appear on your map as a purple question mark. It should say "The Last Enemy that shall be Defeated". Be prepared, it is a quickdraw mission. After that, there are other things such as the Gang Hideouts, Treasure locations, Stranger request Missions, collecting all of the suites, and getting all challenges to rank ten.

After making the transition, there will be a stranger's Task in Blackwater train station.
This is the final quest that closes up the story line.
The mission is called "Remember My Family"
After that, or before it, you can do various things, such as hunting, picking flowers, treasure hunting, stranger's missions, Bounties, gambling, basically anything that you could do as John.
As Axionas said, it's ends in a duel.

Axionas: "The Last Enemy that shall be Destroyed" was the view in which John, emm, you know....
The last one is "Remember My Family"

There is only one mission left and it will appear on your map as a purple question mark. It should say "The Last Enemy that shall be Defeated". Be prepared, it is a quickdraw mission. After that, there are other things such as the Gang Hideouts, Treasure locations, Stranger request Missions, collecting all of the suites, and getting all challenges to rank ten.

What you are supposed to do after John dies is entirely up to you. There is one stranger mission you do and that completes the story. After that you ride around the place trying to achieve 100% completion. John's past life was rough and that showed you how he was, but Jack's life is just starting. So it is up to you to decide if he is the good guy or the bad guy. You may have to complete other challenges to attain 100%.

Go to black water and visit the train station and start the final mission, playing as Jack is about avenging his father (John Marston) good luck fighting the government