While i don't believe the Rangers of the North are by any means essential i would have preferred them to be the relief force over Haldir and his lot any day. Nice as the scene where Elrond deliver's Anduril is, it was a bit of a wasted journey when he had already sent Haldir's lot off in aid- 'Right i've sent off a hundred crack archers and a decent captain to aid him. What? Killed to a man? Right, i better get that bloody sword fixed then- Arwen, ready my horse!' You could have actually combined the two to be honest- replace Elrond's sons in the book version with Elrond himself- would that have been a bit too sacrilegious? I do kind see Elrond's appearance at Dunharrow as a nod to his absent sons. But yes, the Rangers would have been a nice touch, and even showing up before the Battle of Helm's Deep would have been more true to the book than the rather erroneous appearance of some half-baked lothlorien captain and his posse of elves (i think it would also have done more to suggest that Helm's Deep was actually a human victory- the idea that 'only the elves can save us now' is a bit patronising to the valour of Rohan)
The Rangers though are not the characters i would most liked to have seen in the film though. In fact perhaps the people the film most needs (to cover up a bit of a plot hole) is the troops of the southern fiefs, to justify the trek all the way south to Pelargir when it would cut miles off the journey if Aragorn and the army of the dead just legged it along the white mountains, dealt with the forces at the pelennor and then were ready and waiting for the corsairs to arrive. If they were so bold to spend more time explaining the actual situation of Gondor instead of sticking in a shedload of book inaccuracies and unnecessary 'dramatic' additional plot points, they wouldn't leave a plot hole that is (as a rough guess of the difference between the relative routes due east vrs south east and then north) 200-odd miles wide. It would also have given Aragorn a bit more of a claim on the throne of Gondor if he was shown to have the backing of the actual soldiery of Gondor. Then again, to so quickly discard the highly costly (budget wise) Army of the Dead would have been a bit of an anti-climax: there was solid reasoning about using them as the force Aragorn brought to the Pelennor, even if the actual execution of those scenes were pants.
It is something that i've found increasingly niggling that there is never really a point where Aragorn's identity is revealed and accepted by the people, or even the soldiery of Gondor. The way the film is cut together and staged (particularly during the Last Debate scene) makes it seem like Gondor has just unwittingly suffered a kind of coup-de-etat by a tramp and his bunch of hippy drinking buddies. I know this isn't really an issue of an individual missing character, but it would have been nice to have shown either him being gratefully accepted into the city by the liberated troops of Gondor or else have stuck a couple of Gondorian Captains in the background of The Last Debate just to make clear that Aragorn does actually have the blessing of the people he's now ordering a suicide mission to the black gates from. Just a point.
Almost universally i agree with the choice and reasons of the book characters removed, and so coming up with a single character that i feel should have been kept in is quite hard. Gildor obviously was easily removed (though nodded to in the elves heading to the havens while Frodo and Sam are camping), Bombadil and Goldberry would have been 'stylistically tricky' to reconcile, as well as quite a lengthy diversion so i could almost claim i'm glad they were removed (charming though they were in the novel). Glorfindel requires too much explanation for a character with only one real moment, and probably would have made people question why Legolas was chosen for the Fellowship when they had an elven character already nicely established. Erkenbrand is trickier than you might think since his presence would have meant Eomer got more screentime in TTT and therefore could have been more fitfully developed, but on the other hand is a character of very limited consequence, no prior set-up and no later significance (Tolkien left him overseeing the clear-up operation at Helm's Deep). Ghan-buri-Ghan was a clean ommission and doesn't really add anything to the story, as well as being something of a racial stereotype of African Pygmies so meh, i like him but it's fair game he's gone. Beregond was pretty much in the film already (as different as he is to Pippin's helpful guide, Irolas does pretty much fit the bill of Beregond). Bergil and Ingold? Well, they didn't have the time for those characters. The Fiefdom captains are quite interesting- Imrahil serves as kind of a contrast to Denethor, being publically heroic and chivalric while Denethor is far more private, political and shrewd. My own personal choice for Imrahil though would have been too young at the time to convincingly play Sean Bean's uncle (since he's a couple of years younger than him) so it's probably better i can now have my perfect Imrahil added to my mental visualisation of the books now he's at the right age. Forlong would have been a laugh- it's quite amazing the amount of affection a character who doesn't speak a single line of dialogue can illicit. Just having Forlong as a background figure- a Captain of Gondor who had gone to seed would have satisfied me. Hurin, Warden of the Keys could have conceivably been portrayed as one of the Guards of the Fountain Court but obviously isn't essential (could have served the purpose of validating Aragorn's claim though- see earlier), Ioreth provides a nice little comic touch to the book and also is an example (albiet not a wholly flattering one) of a female character in Tolkien's work who isn't either a monster or royalty (Lobelia Sackville Baggins probably qualifies as the former). So yeah, i can't really think of any of those characters who would have really added anything to the film.
To sit on the fence, the one character i think should have been given a cameo is the fox that noses around the Hobbits camping- no talking, just have him do his foxy thing and walk off. I don't think you have Red Foxes in NZ anyway, so it would have been a nice european touch.
Dr Death