Agree that you should build a backbone with the RTW (long and expensive flights) and then add short flight with seperate tickets.
You did not say much about your budget for hotels etc.

Places i have been and i would highly recommend going to are:

South America: Easter Island, Maccu Piccu
Africa: Capetown + Garden Route, Kenya - Mesai Mara (Little Governors Camp)
Middle East: Dubai (2 days are enough), Oman, Cairo and Egypt (is it in the middle east?)
Europe: Rome, Florence, Athens, Paris, Barcelona, Swiss Alps, Munich, Berlin
Asia: Vietnam, China, Myanmar, North Korea (if you can get a visa), Cambodia (Angkor Wat)