1) direct sequel

Working titles included The  Loathsome Lambton Worm and The Wonderful Legend of the Lambton Worm

In 1989, Shaffer wrote a thirty-page film script treatment entitled The Loathsome Lambton Worm, a direct sequel to The Wicker Man, for producer Lance Reynolds. (wikipedia) Hardy and other's didn't like it and it was never produced. (See also Brown's book for the script which he calls The Wicker Man II. Shaffer didn't like Roman numeral-titled sequels.)

"Where The Wicker Man drew from Celtic pastoralism and ancient Greek propitiatory myths dealing with death and rebirth in the cycle of nature, The Wicker Man II is spiced with references to more esoteric European folklore such as Poleviks, which are Russian corn spirits; the Rusalka (image used in script), an erotic spirit which dwelt in trees; and the Leshy, another Russian forest spirit. There are also plates depicting the legend of Faustus….” (Excerpt From: Allan Brown. “Inside the Wicker Man.” iBooks.) A cover used a an old photo of a Kern Baby. The Lambton Worm is an English folk tale.

A commentator from the article below remarked: "One minor quibble. A Rusalka is a water nymph, said to be seen in trees, but not a tree spirit."

An article about it. Wikipedia

An imagined poster for it. Designed by Steven Sloss. That's Britt Ekland on the couch from the movie Scandal.

2) The Wicker Tree

3) The Wrath of the Gods