Apparently this is a Thing; it's got its own page on Wikipedia. It's "a product or fictional work which is similar to, or directly inspired by, another previous work, but (unlike a traditional prequel or sequel) does not explicitly continue the product line or media franchise of its predecessor, and is thus only a successor 'in spirit'. Spiritual successors often have similar themes and styles to their source material, but are generally a distinct intellectual property."
© Haydn Thompson 2021