I have some other ideas but they are more complex, and I don't want to be too unrealistic. Needless to say, they involve different variables (e.g. current wealth of dynasty vs. average wealth of all dynasties, religiosity, average dynasty disposition, number of rogues in dynasty, political positions held). Basically, let's say a dynasty was extremely poor (thus, lacked financial means to achieve their goals), had no political ties, poor average disposition, and no ties to the church or local politics and relationships with a lot of rogues: this dynasty would have more incentive to be aggressive (i.e. criminal).
Each one of these variables could be measured for each dynasty and then given a + or - value. In the above case this dynasty would have a +5 aggressiveness which could then be checked against their relationship with your dynasty. Each level of relationship could be given a +/- value as well. So if they absolutely hated you, they'd have a +10 chance to be aggressive toward you. Basically they'd be attacking you constantly.
That would be a very good system but it does sound rather hard to implement.
I think the attacking is already hidden in their AI it is just not coming out in renaissance, someone stated it has been toned down for beta, hopefully they can easily tone it back up, preferably even higher than before.
I am going to assume for now that the main reason for the AI's lack of aggression is either that or economic reasons. if it is economic, then perhaps you could simply allow the enemy to cheat. If it is on hard setting they could get 1000 gold added to their balance for each building they own, every turn. Also, is it possible to change the price the AI pays for titles but not the player? So the AI could pay half what the player pays for the same title?