I actually posted this on the flanking bonus page but I felt it was completely unrelated to the topic so I am posting it here.
Oh well. Improving AI sounds too much to ask for. Then what about instead of adding flanking we add an ambush feature? Only allow it to be used as a defensive tactic - meaning you can only use it in your own regions.
Once players press the ambush button, their units will become invisible from scouts and region status pages. Also, depending on the region type, the chance of being discovered should change as well. On rural+badland regions, scouts can detect them 40-50% of the time - failed attempts leading to the loss of scouts - maybe not all the time but it can lead to scouts returning with the same information as the region status page. In mountain+forest regions, make the chance of detection much slimmer, something like between 10-25%.
Once ambushing units enter the battle, make the ambushing units hit 1.5-2 times harder and make enemies suffer from significant morale loss while giving them 10-20% withdrawal rate. Or instead of hitting harder, give the ambushing force higher chance to wound nobles?
But if they are detected before being ambushed, make them fight like a normal battle - without the ambushing group knowing they are detected.
To balance things out, maybe limiting how many units can hide at once wouldn't be too bad.
To add more, disabling this option for fortified regions must be added.
