Silly thought, but a Wolfhound is huge, is gentle to most, but can be stranger-wary, not super hard to train (though their teen period can be fun...), and definitely intimidating to anyone with bad intentions. They are not guard dogs by any means, but most will bark when someone comes to the door or up the driveway. They are also super loyal to their person/people and would defend bravely if in a dangerous situation.
It's not the breed for everybody, but might be something worth looking at. They are good with kids, other dogs, and often smaller animals like cats (though some lines are more prey-driven than others). They need a good run here and there, but are not terribly active dogs. Their biggest flaw is that they are short-lived
Anyway, just an idea.