Ever since it was announced that The Hobbit was to be split into two movies, there has been a lot of discussion that the second film would continue beyond the book, conflating appendix material in order to connect the films to the larger trilogy. There is a lot that can be done with characters who had appeared in the three The Lord of the Rings films (though John Rhys-Davies has already confirmed that he will not be returning to play his character, much less a dwarf of any kind). Coming Soon spoke to Viggo Mortensen about the possibility of returning to play Aragorn in the second film:
“I think they’re just worried about shooting the first part of that movie which doesn’t involve my character,” Mortensen told us during an interview for his upcoming movie The Road. “My character is around at times–in the middle, but it would only be if they made a second movie or connecting movie that connected ‘The Hobbit’ to ‘The Lord of the Rings’ that I might appear, which would be great.”
No doubt just about every actor from the trilogy who makes sense here will have to confirm or deny their involvement with The Hobbit. But I wouldn’t want a character to appear just for a reunion. They would still have a major and convincing part in the film, one that makes sense according to their character arc.