Wouldn't work, who is John gonna fight?
There needs to be an Enemy and people won't like fighting constant flood all the time, it wouldn't be bad but ya'know.
I think it is something with a twist.
Could be Forerunners
Could be Halsey, and the Remain S-II's and S-III's.
Just have to wait till Pax when they release Gameplay for the Campaign and Mulitplayer.