Atheism versus Agnostism Is the question about the existence of God, or the nature of God?
If you ask, are there things which are Real which I am not able to imagine, the answer would have to be yes. Trivially, the history of science illustrates this (paradigm shifts). So if you base your 'negative affirmation' (God does not exist) on your own inability to conceptualize God, well, that is a personal limitation. I saw a video where Dan Dennet made exactly such an argument.
If, on the other hand, we were asking, what might be the nature of a God that really exists, that is a much more interesting discussion. I would say that atheism is...unnecessary.