fantasy vs imagine

fantasy is a noun but imagine is not a noun.

Word	Noun	Adjective	Verb	Adverb
fantasy	Yes	No	Yes	No
imagine	No	No	Yes	No

As verbs, imagine is a hypernym of fantasy; that is, imagine is a word with a broader meaning than fantasy:

fantasy: indulge in fantasies
imagine: form a mental image of something that is not present or that is not the case

Other hypernyms of fantasy include conceive of, envisage, ideate.

fantasy (noun)	imagine (noun)
imagination unrestricted by reality
something many people believe that is false
fiction with a large amount of imagination in it

fantasy (verb)	imagine (verb)
indulge in fantasies	expect, believe, or suppose
	form a mental image of something that is not present or that is not the case