spirit world vs imaginary place

spirit world and imaginary place both are nouns.

Word	Noun	Adjective	Verb	Adverb
spirit world	Yes	No	No	No
imaginary place	Yes	No	No	No

As nouns, imaginary place is a hypernym of spirit world; that is, imaginary place is a word with a broader meaning than spirit world:

spirit world: any imaginary place where spiritual beings (demons or fairies or angels or the like) abide
imaginary place: a place that exists only in imagination; a place said to exist in fictional or religious writings

Other hypernyms of spirit world include fictitious place, mythical place.

spirit world (noun)	imaginary place (noun)
any imaginary place where spiritual beings (demons or fairies or angels or the like) abide	a place that exists only in imagination; a place said to exist in fictional or religious writings