I really do suggest that AI be upgraded for Stronghold 3. Since strategy games have AI as the soldiers, and AI as the PC controlling the castle as if it were player, I'll list my ideas for upgrades separatley for less confusion.

Bots (Soldiers, troops, etc)

I'd allow for some options to turn off auto attack for some things. When I lose a game and I tell my lord to run, he always stops to fight anybody he sees and gets himself killed unless I click 1000 times to keep telling him to go


Artificial Players (What?)

If you play on a map like Grasslands (Stronghold 2) you'll notice that the kind has their own style for a castle, but it's always the same. Though each lord has their different style they tend to only have one style each. Play a few rounds against someone like the King or Olaf and you'll notice they build the exact or near exact same castle every time.

Another quick thing to the lord AI is to possibly change the difficulty on them. You can change the advantage by changing the coins, but that's all it does is change who gets more coins to start with. Why not change the pace they build, attack, produce troops, and all those sorts of things as well?

Being able to turn off Auto attack would have saved me many troops over the years, but it isn't very realistic. I know if a guy came hacking at me with a blade i'd stop to defend myself i'd still welcome the option, for multiplayer if anything.

I'm not sure how feasable randomly generated castles are, simply because the presets are designed to fit in with all maps. In theory a brilliant concept but whether it is technically possible in FFs eyes is another thing, I feel if it was it would have been done already.

What FF can do easily (if the SH3 AI editor is anything like that of SH2) is increase the amount of predetermined designs each AI has to choose from and like you said increase the difficulty. I would like to see AI opponents build up a force based on the players overall army count and unleash their troops when it is determined the fight is 50-50. Weaker AI would have more difficulty calculating this.
Just my take

To the auto attack comment if you think about it that way, your right, but if 20 people with swords charged you would you stay and die or try to dodge and keep running whenever they caught up?

About the AI castles, there are a few ideas I had. Firefly could come up with a few different castle designs for each person instead of just one, or they could actually allow the mapper to choose to use the default designs, or spend the extra time to create their own so it's map-based. Or they could even do both listed.

Adding on to the first idea though since they may be too busy to do it themselves, they could quickly create a small sandbox program, sort of like SH2's map editor but stand alone, and they could open it as if it were an open beta and allow some people to submit different designs and vote on some ones to be used in the game.

I really like all the idea's presented here regarding Artificial Players. I'd also hope the AI troops use a variety of tactics and use the land! It was so annoying making a beautiful map and having the AI use the SAME path to attack you making the map less fun and very predictable.

Another problem to fix was if you left a very small hole to your keep and didn't enclose it, no matter how far or heavily defended it was, the AI would run straight through to get to you and not try sieging you where you're undefended.