Fantasy is fictional -- not real or something you are imaging. "My fantasy is to live on Mars." Fancy is dressy, elaborate, not plain. "She wore a fancy dress on her date." or "The diner plates had fancy gold designs around the edges."

There is a verb and an adjective version of “fancy”, The verb form means “to like something”, The adjective means “elegant or ornate or maybe gaudy.” “Fantasy” can be like science-fiction or Harry Potter, but “fantasy” can also be something like a dream or a desire.

Consider a... This part of a sentence is used to express a choice, preference or opinion. Ex. She would consider a better job. Consider as... This part of a sentence is used to express a comparison of something similar, literally and figuratively. Ex. Quitting her job could be considered as an alternative. All of that to show you that "a" and "as" are independent from consider, but depend on what is after them.