I have seen many books showing designs for "underground" houses which use concrete roofs. You could just build a taller one of these and still make a conventional lumber 2nd floor deck. If you had a poured concrete roof and walls you could use a timber floor deck with no obvious sacrifice in strength. What about all of those office buildings and college dorms and parking garages that use concrete floors? I had a college dorm room with what looked like a waffle for a ceiling. This was obviously a form for a relatively lightweight reinforced poured concrete floor. Just keep looking for a contractor that knows what you are talking about and get a bid from him. If any contractor for any job tells you it can't be done, looks at you like you have 3 heads, or starts scratching his head, then move on and find one who knows what he's doing.
I think what you need to do first is to get this structure designed, engineered, and drawn. Any architect that would confidently draw this would probably know a contractor that could pull it off.