According to Yahoo! writer Daniel Wood,
There are five characters from The Lord of the Rings who should have reprised their roles in The Hobbit: Aragorn, Gimli, Denethor, Arwen, and Merry, Pippin, and Sam, who for some reason all count as one. (I know they're only halflings, but wouldn't that still make it one and a half?) Anyway, he gives some pretty interesting reasons.
Directly from Yahoo! writer Daniel Wood:
"I thought I'd get Aragorn out of the way first. Viggo Mortensen's Aragorn has become somewhat of a major cult icon thanks to his role as Aragorn and when rumours that he'd be making a cameo in 'The Hobbit' began to surface everyone, myself included started getting a little excited. However Viggo shot those rumours down by saying that it didn't make sense for him to return. However Jackson is bridging the gap between 'Lord of the Rings' and 'The Hobbit' using extra source material from Tolkien's writing, therefore there are one or two scenes that Aragorn could return for in order to ease the transition from 'The Hobbit' to 'Lord of the Rings'. I'm still holding out hope that Aragorn will in fact be making an appearance."
[Comments: This would be awesome, but unrealistic. Aragorn was only a toddler when Bilbo left on his unexpected adventure.]
"Gimli's father Gloin is one of the dwarves in Bilbo Baggin's group that is trying to reclaim Erebor. Gimli also would have been alive during the events of the Hobbit, so if they're going to include Legolas in 'Desolation of Smaug' why not include Gimli. I'm thinking a little scene at the end where Thranduil (Legolas' father) and Gloin meet alongside their prospective children, hinting at the bromance that is to come in 'Lord of the Rings'. It's not necessarily canonical, but it'll be a nice little precursor. If this were to happen though, the role of Gimli would need to be recast."
[Comments: It would be amazing if we could see Gimli return in The Hobbit, but sadly, it has been confirmed that John Rhys-Davies will not return to Middle-earth.]
"Everyone will remember the mad ruler of Minis Tirith who knowingly sends his son to certain death. Denethor was a hated and conflicted character, but most important he was a human man. 'An Unexpected Journey' was the first film in the 'Middle Earth' universe to not include a man at all, and whilst it's difficult to truly hate a CGI-orc it's much easier to hate another person, especially one that makes as many questionable decisions as Denethor does. Give him a small appearance, reiterate to the audience he's not okay and then move on."
[Comments: I agree, it would be nice to have a human villain you can hate. However, the same problem arises as with Aragorn: Denethor probably was not even born when The Hobbit took place.]
"Liv Tylers elven love interest was alive and kicking, living with her grandparents Galadriel and Celeborn. We've already seen Cate Blanchett's Galadriel and as most people are aware the film does need some more female faces. Jackson has already controversially included Evangeline Lily's Tauriel to add some feminine charm, but that seems silly when he has Arwen right there. Jackson should have Arwen accompany her grandmother during one of the council scenes, just as a nice Easter egg for fans."
[Comments: Plausible? Yes. Arwen was probably only the young age of a few centuries, possibly close to the age of Tauriel. However, what role could she possibly play? She could add nothing to the story when Aragorn is only three.]
Merry, Pippin and Sam
"I don't know why but I have this image in my head right at the end of 'There and Back Again' of an aging Martin Freeman as Bilbo Baggins overlooking Frodo playing with some other Hobbit children, who are of course Merry, Pippin and Sam. I envisage some sort of knowing voice over depicting how little the young Hobbit knew about how important he would be. I think this would be a suitable ending."
[Comments: This would be totally awesome! If you haven't already noticed, I'm crazy about hobbits. I was disappointed these three blighters didn't show up in the prologue of The Hobbit: An Unexpected Journey, and am still hoping they show up in the epilogue of The Hobbit: There and Back Again (If there is an epilogue).]
Source: Yahoo! Contributor Network