How do you imagine your courier eventually will die?
For mine, he sets up a new nation in the Mojave and all goes well. However, in creating it he did things which he regretted (destroying BoS bunker and the Great Khans). I like to imagine eventually this comes back to haunt him.
Maybe once he's stepped down as leader one day Veronica arrives on his door - she has learned who ended the Mojave chapter of the Brotherhood of Steel.
She is torn: she likes the new nation created and helped play a part and doesn't want to see it crumble, but still wants justice for her family. At the Courier's home she takes both him and herself down in an explosion.