Didn't think the Dementors or Black needed to be any more menacing than they are--the way I see it, Potter's biggest nemesis was himself.
The key for me was that Boggart (sp?) thing--it's more than just a creature that feeds on fear, it's a device to expose one's innermost thoughts. Potter was most afraid of the Dementor, who represents authority, ergo: he's afraid of having to go against authority in a more open and rebellious manner. Yeah, he's bent the rules once in a while before, but setting free Black and the hippogriff (come to think of it, their stories sort of run in parallel--the hippogriff's fate is a metaphor and foreshadowing of Black's) is in defiance of established law. He's taken a step beyond; hasn't been caught yet, but he's taken one step closer to being every bit of an outlaw as Black himself.