Time travel based FPS

Hello everyone! After some long time lurking, I have gathered up the motivation to work on a new game. So far though, all I have is the concept. I am not completely new though, I have experience in python, c++, c#, Unity, and modeling in Blender.

The Concept
The game is a two player competitive FPS. One player plays as the assassin, the other as a time-traveler. The assassin's turn comes first. He, on his own tries to kill some important figure. I should mention this takes place in a modern setting. Once he finishes, it becomes the time-traveler's turn. The assassin carries out the exact same motions as he did when controlled by the first player. The time-traveler follows and attempts to prevent the assassination. The catch is if something is different the second time, (the assassin sees the time-traveler, a door is shut, an elevator at the wrong floor, etc) continuity is broken, and the first player regains control. Now both players are playing at the same time. The game ends if either are killed, or the assassination goes through.

So there you have it. I am currently modeling some guns and other props. In the mean time, tell me what you think. Should I change anything major, or should I scrap it altogether? You tell me.