I’m not understanding the “per category”, “per post” title.
At least I’m failing to see how letting members set their own titles at that level could be of any usefulness to others.
But I do think custom user defined titles based on Trust Level could be a good idea.
That is, for Trust Level 3 members, instead of needing an Admin to set a title or choosing among those made available because of receiving a Badge or <none>, being able to enter some short custom text could be a nice perk.
But then, custom user fields already fill that to some extent.