This has been suggested before, and rejected. Link to the thread. Link to the Rejected List
However, if you think about it, assassin is a form of a secret mission. The mission cards had missions "Eliminate all the Red Armies from the game".
Missions applicable to all maps is difficult, hence we only have "Assassin." But, we do allow for individual maps to have Missions and Objectives, and a number on the web site currently available do so.