I would have loved to have a negotiation quest in the game. You could sign a new treaty (after taking out House of course) allowing New Vegas to be annexed into the NCR and be taxed but allow protection in the land (both NCR soldiers and securitrons) and have up keep costs on the city. (Repair, Food, Water, ect)
Something like this would never be canon (NCR would be way to over powered) but it would have been nice for those who liked the NCR but wanted to make Vegas a more powerful place to protects it's people.
Obviously this couldn't apply to the other main factions as House is set in his ways and CL don't want Vegas as it stands so NCR makes the most sense.