Otherwise you may end up copying an idea unknowingly.
It's hard to come up with something really new these days.
To create a really original game you should create a game that doesn't fit in any known category like First person shooter, Role playing game etc.
Also thinking of new original themes is very hard these days, look how may WW2 games there are for example.
So it's probably 99% chance you end up copying something anyway in gameplay.
EDIT: By now all I thought of was a 3D turret game, where enemies fly around you and you have a 360 view with your mouse-controlled aim. Does it sound like a contest winner? =/
This idea is not new, google up the game Incoming from Rage. You have turret levels and levels where you end in another verhicle.