The simple fact you ask such questions is great however shows you have years to go before you hold the proper knowledge to even consider breeding anything, let alone an animal and program of which can turn into wash quicker than a blink of the eye..
When you combine gameness, guardian and high amounts of prey, sophistication, harder animals mentally and physically.. Genetics is your savior as without the knowledge of not only your foundation roots, foundation themselves and your clear visions, you will end up with cold dogs, unfit to be worked, unsound mind or body, etc.. The margin of error compared to that of success is paper thin.
Another question you must ask is what do you see with this vision that doesn't already exist? Very rarely does what you seek not already out there and it has ALL been done before thus you must learn from what has been done.. If it ain't there today you have to break down, genetically, what went wrong.. Which again, takes years to fully comprehend and learn what to look for so that you do not breed yourself back into the same corner of which killed off the animal previously.
Another thing is that as a breeder you must breed for yourself, not for someone else, thus you have to have proper financial stability, available funds, ability to use ALL stock off your yard.. It is a job of which few really have the full access and time for.
You will also notice much as FH has already pointed out, working stock Mastiffs AKA Bandogs mimic in appearance and mind that of Game Dogs, Bulldogs... APBTs.. GEMS of past and present, a reason for this as genetically cross and intertwine far more than the common person may truly realize.
Old English Dogge, rooted directly towards Danish Hound, War Mastiffs, modern wash that is of Great Danes.. One of the oldest of genetic foundations..
War Mastiff of Neo..
Direct lack of fear regardless of weapon used, to the point sourced mind of gameness.. Tested, trialed and proven by man over centuries.. The ability to take down what needs to be, threat is no object.
Notice the near hound like structure..
Bulldog in traits, Mastiff by structure.. Put in basic
Old as you get of documents, what do you see? Notice the length of legs, depth of chest.. Wind, ability to hit long range targets with less ground noise making subjects unaware until contact.. Pure, instinctive.
Companionship with eyes and ears always on sight and targeting threats beyond that what a human may detect.. Man or animal, eager.
Again we go back to that of Bulldog.. A reason.