Isn't that the plot for the Scarface game?
As for me, well I have two ideas for a setup scenario.
One is perhaps a little to much like Fallout 1 or 2's setup (find McGuffin(s), and defeat foozle), while with the other I would like to do something really different (still foozle at the end though, can't escape that).
In the first one, the player unlike Fallout 1 and 2 isn't a Chosen One.
Rather he or she is more of a down on his or her luck slob who comes to New Vegas for opportunity and riches, perhaps looking for a place to belong, or being someone in exile.
I am not sure of how the player gets a Pipboy, perhaps he or she is a former Vault Dweller, found it on the corpse of a Vault Dweller, or perhaps won it from a Vault Dweller.
The Pipboy might be involved in the storyline or not.
After going through the setup part the player becomes part of a sort of treasure hunt, going in search of something special while competing with other factions (perhaps included BOS and NCR soldiers).
At some point the player can perhaps choose to join one side or set up both sides against each other while going for the mystery price.
In the second one the player is part of a Vault community or some village with a reasonable level of technology.
One day a group of power armored soldiers, I am using the name Circle of Steel as a temporary placeholder, attacks the city, kills and wounds several defenders and civilians and steals important items that are crucial for the existence of the community.
With no other option the leader(s) of the Vault/town sends out several selected people, including the player, who must go out to find the stolen items or replacements in order to save the community.
While the player is pursuing this goal he or she might also investigate the attackers; where they came from, what they are up to and where they are heading.
At some point this becomes crucial as more than just the Vault/town's existence depends on it.