Better team AI. In Vaegir and swadian armies, that's all that's needed. When it comes to my forces vs almost identical opposing forces, i always win with very few losses because i get my guys to stick together, and the enemy comes in a very sparse configuration. As soon as a guy engages in a 1 vs 2 fight, his fate is pretty much sealed.
River pirates and bandits should be kept the way they are in terms of AI. A bunch of ragtag ex-convicts with no sense of teamwork. Foragers and deserters somewhat the same, but with some battle sense at least.
Perhaps the leaders, the knights and such would be the ones who actually tactically lead their army, issuing commands such as 'charge' and 'follow me'. Once you off him, you can treat his soldiers as just another group of disorganised bandits.
It would be nice if you occasionally saw knights unhorse of their own accord. So they might actually know that when he's the only guy on a horse, it's better to get on foot and fight rahter than pulling a Borcha.