I would explode with excitement if liberty prime was a companion and supposedly someone is coming back from fo3
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The only people from FO3 to make it from the Capital Wastes to New Vegas, (at least those would would make any sense, survival wise), would be;
Charon .... but why would he show up to be your companion when he's got a sweet deal going where he's at.
Harkins .... see Charon's excuse above.
Desmond .... He might actually have a reason to show up in New Vegas as part of his on going "Big Game" plan. But as for him being your companion... HAHAHAHA... ok sure, he'll "allow" you to hang around him for about as long as it take you to finish a couple of particularly unpleasent jobs for him. Then he'll kick you out the door with a few nice parting gifts. Sorta like being used by Fisto, only with more entertaining diologue.
Fawks ... he could make it to New Vegas, but why would he bother ... same as Charon's excuse.
Nobody else could make it from DC to New Vegas without having at least a small army to escort them, (or being dumped off by space ship Zeta). Hopefully no one does. I'd only want to hear about anyone or anything from FO3 in FONV by way of myths and legends.