Well we all kind of knew this was coming. In some ways, campaign co-op would be nice, but in many ways it will just ruin it. Mass Effect has always been about your personal Shepherd, and if there is somebody else there it kind of spoils it unless they are just literally a gun, but then what happens in the non-combat zones?
Naughty Dog had the same issue with Uncharted, whilst the actual gunplay would be fine with 2 players, 2 player exploration and platforming doesn't really work unless specifically designed around that, so that is what they did.
If, however, they just take the focus away from Shep, and maybe see some of his allies in a combat mission, and let you mess about with nice biotic powers or something... I think there is potential, but it has be entirely away from the main story, and have nothing to do with your Shep