Sharing your fantasies is a very important part of communicating both your desires as well as dislikes. Dislikes are not necessarily limits. Knowing what you don't like can be useful in pushing those limits and exploring things that may be exciting that you never expected. As far as topping from beneath, unless you are rigidly specific about following your fantasy; that is using it for a script to follow, I wouldn't consider it in that vein at all. Even then, following a script may be used as a reward or if your master enjoys it as well, be the basis of a play time. Fantasies are just that; and trying to create them in real life may be either disappointing or a tremendous thrill - for both of you. Demanding that he satisfy your fantasy might be construed as topping from the bottom; but sharing them....hardly.