So I'm working on putting together an essay that captures my thoughts on Willow and its relationship to SW, as well as Lucas's various influences while writing it. By no means all of it is relevant to a SW forum, but I'll excerpt some interesting bits.
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Willow's species, the Nelwyns, were obviously modeled on the Hobbits from The Lord of the Rings. It appears that, early on in the writing of the film, Willow's best friend Meegosh was going to be a more important character. His role was probably reduced because he proved to be too obviously similar to Tolkien’s Sam Gamgee.
At one point during the writing of the original 1977 SW, Lucas considered having all the natives of Tatooine—including Luke Skywalker, Owen and Beru, and Ben Kenobi—be played by little people, with everyone else cast as an average-size human. The idea was to make Tatooine into a space-fantasy equivalent of the Shire in LOTR, but it never got very far.
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As well, I mentioned in another thread that the idea of a "magic tree" guarding the entrance to the Force cave on Dagobah in ESB was almost reused in Willow.
Apparently in early outlines for Willow, there was a scene with a tree growing above the mouth of a cave where a fierce dragon lived. The tree itself would have been ordinary-looking on the outside, but with an inner core of solid gold (a reference to Kurosawa's The Hidden Fortress, where Princess Yuki's golden treasure is hidden inside sticks of firewood).
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Also, as far as the prequels go, you can see a LOT of influence on their art direction stemming from the French artist Moebius's concept art for Willow. Those designs, in turn, were influenced by Lucas wanting to reuse ideas from the early drafts of the OT that never made it to the screen.
Observe:
This is Moebius's concept drawing of Madmartigan. Although he clearly resembles Robert E. Howard's Conan the Cimmerian, the hairstyle incorporates Lucas’s love of Asian costume. It’s long in the back, but with a shaved portion just above the forehead. This style blends the shaved pate of a Japanese samurai chonmage with the shaved fore-crown of Manchu-era Chinese men’s hairstyles.
The dark hair also boosts Madmartigan's resemblance to Toshiro Mifune, who was one of the two major influences (the other being Flash Gordon) on the appearance of protagonist Annikin Starkiller in the very first rough draft of The Star Wars.
Except for the shaved forehead, this hairstyle survived intact for the final film’s Madmartigan…
…and was actually recycled in early TPM concept art of Obi-Wan Kenobi.
Madmartigan's name is a combination of Tolkien's "Aragorn" and "Mad Max" from the eponymous film.
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Here's Moebius's take on Sorsha.
Note her blonde hair. At this early point in production, Sorsha was envisioned not as a human like in the final film, but rather as a half-Elf. The final film's fully human Sorsha has red hair, much like Princess Leia Aquilae in the rough draft of SW 1977.
The white mask Moebius gives Sorsha here, which combines geisha makeup with the horns of an oni, invites comparisons with another character from the rough draft of The Star Wars: Prince Valorum, a Sith knight who switches sides near the end of the script and joins the good guys. At the end of the story Valorum is one of the heroes seen sitting in honor by Queen Leia’s throne.
The character arc of Prince Valorum would later evolve into the redemption of Darth Vader/Anakin Skywalker. Darth Vader’s helmet, in turn, would find a descendant in Sorsha’s oni mask. However, like Valorum, Sorsha survives the film.
Even as late as the “third revision” script, Sorsha is described as having a face-concealing helmet. This idea would later be transferred to General Kael, a villain who is not redeemed.
I also suspect, given the origin of Madmartigan's name, that the white mask in this portrayal of Sorsha is inspired by the villain of Mad Max 2...
...who is known as "The Humongous." The half-Elf version of Sorsha would indeed be "humongous" standing next to her full-Elf kindred.
But that’s not all. The end of the rough draft of The Star Wars was clearly inspired by the ending of Kurosawa’s The Hidden Fortress, where Princess Yuki is revealed as the beautiful ruler of the Akizuki clan in exile. In this scene Yuki is shown wearing white geisha makeup, clearly visible despite the black-and-white cinematography.
So that idea passed down to Sorsha’s hybrid geisha/oni mask, and then finally…
…to Queen Amidala in The Phantom Menace.
As well, Sorsha’s shaved-head-with-topknot hairstyle in the above concept drawing was reused in TPM…
…for the briefly seen Aurra Sing, an extra in the Podrace scene.
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King (later General) Kael, the monstrous second-in-command of Bavmorda’s army. In Moebius’s drawings Kael is an inhuman creature, and he is still described as such in the third-revision script. However, in the final film he is merely human, though he wears a face-concealing helmet like that originally planned for Sorsha.
Note the forehead tattoo and horns that Moebius has given Kael here. These motifs would later recur...
…in the character of Darth Maul, who like Kael is merely the henchman of a greater villain.