The Lord of the Rings is the epic fantasy to end all epic fantasies — single-handedly creating a massive industry for the genre in the past two decades. One might even say that Game of Thrones owes at least part of its popularity to the benchmarks established by LOTR.
The films are spectacular, with storylines, characterizations, and visuals almost matching the skill employed by J.R.R. Tolkien in the books. There are a few characters, though, who are not as active in the story as they should have been (and even if they are, fans really wanted more of them).
Isildur
Isildur is a crucial character in the early history of Middle-Earth, being one of the founders of the House of Gondor along with his father, Elendil. In fact, it is he who first plants the White Tree that still flowers atop Minas Tirith. The movie just shows his role in the War of the Last Alliance, where he manages to cut the One Ring off Sauron's finger. However, he refuses to destroy it, consequently getting himself killed.
It would have been interesting to show audiences how Isildur comes to Middle-Earth in the first place — the story is packed with storms and lightning and arrogant kings getting themselves trapped underground by the might of God.
Durin's Bane
The Balrog known as Durin's Bane is a terrifying obstacle, preventing the Fellowship from crossing out of the Mines of Moria. Its resultant battle with Gandalf is only shown in tiny snippets, lasting for a few seconds before the Wizard ends up winning by "smiting his enemy against the side of the mountain."
Durin's Bane deserved a little more courtesy than that; perhaps in the form of a slightly longer fight in which each opponent displays their respective abilities in an explosion of CGI fireworks.
Gandalf
The OG fantasy wizard, Gandalf, has a considerable amount of dialogue in the films, but he spends most of his time acting in an advisory capacity. This is frustrating because he is meant to be an Angel of God (known as a Maia), sent from beyond the Sundering Sea to help Middle-Earth against Sauron.
Instead of suggesting solutions in the form of unnecessary puns, Gandalf should have been shown taking the initiative. Audiences would have loved it if he had entered the fray more often than he actually does in the films.
Galadriel
The oldest of the elves on Middle-Earth (save Cirdan), Galadriel boasts of an unimaginable power. The scenes in Lothlorien are incredibly beautiful, enchanted by the Lady of the Wood, but also rather short.
One addition fans would have enjoyed is a flashback explaining why Galadriel's gift to Gimli — three hairs from her golden head — had a higher impact than the history of the One Ring itself. However, she chooses to "diminish, go into the West, and remain Galadriel," instead of taking the One Ring when offered by Frodo.
Elrond
Elrond, played to hilarious perfection by Hugo Weaving, is an accessory character. He provides the hobbits with shelter, establishes the Fellowship, and agrees to let his daughter marry Aragorn, a man. Fans would have loved if Elrond's character was given greater depth via a retelling of historical events.
For instance, the fact he is the son of Eärendil, the starlight that saves Frodo and Sam from Shelob; and that his twin brother, Elros, chose to become a human ruler from whom Aragorn descends in an unbroken line spanning many generations.
Treebeard
Treebeard is arguably the best part of The Two Towers, treating the hobbits (and even Legolas, who is two thousand years old) like children. He helps the Fellowship by calling all the Ents together for the first time in centuries, encouraging them to storm Isengard.
What audiences would have appreciated most is Treebeard's backstory — nobody knows where Ents come from, how old they are, or why they exist in the first place Most importantly, where are all the Entwives?
Gríma Wormtongue
Gríma, the slimy snake of Saruman, actually had a rather important part in the story during the events of The Fellowship of The Ring. Apparently it was he who revealed the location of the Shire to the Nazgûl, who had no clue about it save for Gollum's two clues: Shire and Baggins.
This fact does not make Gríma seem any less despicable, and should have been added to his screen time. Disgusting creatures like him only increase interest in the story, because fans tend to await their inevitable punishment with extra eagerness.
Éowyn
Éowyn was given her moment in the sun, when she kills the Nazgûl leader, the Witch-King of Angmar, right after delivering one of the most badass lines in cinematic history, "I am no man!" Nevertheless, it feels like her character was only developed towards this purpose, as in most other instances she doesn't really seem to wield much power (especially against Gríma).
Watching scenes involving Éowyn's playful interactions with Pippin and Merry, and her face-offs with her brother, Éomer, might have added that bit of feminist spark that was sorely lacking throughout the trilogy.
Saruman
Saruman received a hefty amount of screen time for a secondary villain. Nevertheless, he is present in this list because his death (by falling) was completely unsatisfactory, and negates the real story that Tolkien came up with.
Everyone would have been shocked when the hobbits return to find the Shire under the tyrannical rule of a criminal overlord known as Sharkey (who was actually Saruman). Shock would have soon turned into mirth at watching the nasty old Wizard lose once again, getting his throat slit by his own cretinous servant, Gríma.
Sauron
As the creator and wielder of the One Ring, Sauron (the physical form, not the burning Eye) gets nearly no time for himself on screen. This is patently unfair, considering that the story is literally set around him. In fact, there is a deleted scene from The Return of the King (2003) that depicts Sauron as a beautiful being composed of light.
This was a form he had chosen in the past, opting to go by the name Annatar, The Giver of Gifts, in order to trick the elves, dwarves, and humans into forging the various rings of power. Retaining that scene would have actually pleased the fans rather than cutting it out entirely.