The return of a companion could always be cool, as long as it's done within reason and a smooth transition. For instance if the return of a character means they're old as hell, then that's probably pushing it.
The return of ED-E wouldn't be so smooth because the game endings imply ED-E stays with the Courier, so if ED-E returned then people would ask what happened to the Courier.
Cass, Boone, Veronica and Arcade would all have to age and three of those four have ties to the west, so if we continue moving eastward, that wouldn't make much sense.
Rex, Lily and Raul could always make a return. I wouldn't mind seeing Raul again. Hopefully as a Vaquero.
I don't think of the FO3 companions as being super-interesting, so I'm not dying for their return. Charon was too quiet, Sarge was just a program and Fawkes got old fast.