A lot of things are "conjured from the imagination" before their existence is known. That's the part of science that we call "forming a hypothesis". We can't test the hypothesis until we conjure it. We can't know if it's X or aX before we test it.
Sorry, but forming a hypothesis and conjuring things from the imagination are two different things. Look up "Scientific Method"
If/when you do conjure something from your imagination, what do you call it? If it doesn't exist, what do you call the absence of it?
You don't call it anything other than a notion.
Certainly, the terms are needed whether they use the prefix "a" or are formulated some other way:
My aunt could eat "abananas" - i.e. anything but bananas. (She was allergic to bananas.)In the seventeenth century there were "aautomobiles" - i.e. no automobiles.Living organisms can be divided into human and "ahuman" - i.e. non-human.
Sorry, but you are not using those terms in the same way atheist is used. Try again.