I think it would also be nice if we could designate criminals at will, as this would fill a gap that's in the game presently. As an example, I'm sure we've all had a certain noble that mandated or demanded certain things that didn't get delivered, and then they had a Champion military dwarf or Legendary Armorsmith chained to a wall for 130 days or hammered to death for it.
My freshest example was a Legendary Marksdwarf that was chained up right before a major, crucial military operation, and ended up going insane because she couldn't reach her baby from the jail. She refused to eat or drink and went melancholy shortly after being freed.
The whole fort got a bit upset that she went nuts and died, and several more military dwarves couldn't handle the additional strain of losing a close friend on top of the losses and carnage they'd just witnessed. We lost four highly trained military dwarves because my Baroness didn't want us trading away scepters. I could've solved the situation drastically by not only removing the chain but also firing the whole fortress guard so they stopped chaining her to every single restraint in the fortress, but that wouldn't have been Fun.
So, fort gets devastated because of a broken mandate. Naturally that was the last straw for me and I locked the Baroness in her room where she eventually died. Others would've done something clever with magma and levers. But you know... That's all kind of cheating isn't it? How can the whole dwarven society agree to deathtrap their nobles without a trial, without the fortress or royal guard being involved or caring one way or another, and with the dwarf in question not even trying to resist?
Yeah it's very Dwarfy to fry your nobles with magma, it's wonderful and all, but variety is the spice of life and some may want more immersive, realistic options for dealing with these situations.
What would be better is if I'd decided right then and there that my dwarves are obviously upset and will demand justice, and then I designate the Baroness with three counts of Dwarfslaughter for causing the deaths of multiple valuable dwarves over a stupid mandate. Or 1 x False Imprisonment + 23x Disturbing the Peace + 10x of restricting vital business -- this could be simplified actually by just using the Moral degrees. IE, one count of Punish_Capital because I just want the b***h gone so we can trade masterwork scepters in peace.
The main benefit is that more players will use the justice system if it can actually help achieve their goals, and it's less contrived and more realistic, also more unpredictable and interesting than just locking a door to starve a dwarf you don't like. You still have to deal with any negative thoughts from the punishment being carried out, so over-using this feature would have consequences. Consequences that are Fun.
There'd still be a challenge too, as a dwarf isn't guaranteed to die from a punishment when you want them to, they may even go berserk from the mistreatment at an inopportune time or throw destructive tantrums. If they have friends in the fortress guard they may even be let off, or a guard could get very upset if they have to punish a bunch of their friends. Letting us designate criminals could lead to the unintentional destruction of our whole dwarven society stemming from our desire to control them, and that's Fun and epic.
People could run challenge fortresses where dwarves must be punished horribly for obscure or harmless actions, or failing to show proper obedience to their all-seeing overlord. That is, they can run a despotic city-state without having to be randomly assigned a noble that wants Steel furniture on a fluxless map and demands that no granite items are exported on an all-granite map.