What is legal is straightforward, but what is 'just' is fluid and should be based upon changes and nuances in society. As AI lacks empathy and sympathy, it then lacks a certain element necessary.
I like a jury of peers. But think isolating them would be good if they had access to legal council to help them through the process but with limits to not allow influence by council. (that might be a great place to implement AI)
I also think there should be 3 judges to a case. They must confer and make majority decisions. That takes away the chances of one bad/biased judge having too much power or influence on the outcome.
The other aspect I like in our system is the presumption of innocence and the state must prove guilt. Many countries have the opposite, whereas the accused must prove their innocence. I like our default stance.