To expand the raiding concept:
1 - To make it work, youd need to borrow the HoI3 naval concept of spotting/detection/mission stance, so that battles do not immediately occur as soon as two armies are in the same province. Smaller/faster (cavalry) armies should be much harder to spot. Effects ought to include relationship to the province (friendly/hostile), leader skills and stance. If an enemy settles down to siege in a province, it should be very easy to spot regardless of other factors (obviously).
2 - Pillaging needs to be possible even when the castle has not been taken - castles/fortifications ought to have an effect in that larger castles ought to limit the damage/loot potential by allowing more of the portable wealth of the province to be secured behind strong walls. Pillaging ought to be an effect over time: spend X days pillaging to generate Y return, with the risk of being spotted during that time and suffering combat penalties due to your troops being dispersed and disorganised, loaded down with loot. The decision to pillage ought to be a risk and the player needs an increasing incentive to think "Right - time to run before I get caught". Ought to be similar to the feeling in Mount and Blade when youre looting a village with a small force and hoping youre not interrupted by a larger group whove spotted the smoke.
3 - Newly levied troops will need to begin at full morale, to allow them to immediately engage hostile raiders. Or, the levy could be split between the smaller noble retinue of semi-proffesional nobles and soldiers who would always be available year round to fight - and therefore would be the troops used to raid and defend against raids - and the larger levy of non-proffesionals who would be raised to siege castles and fight large battles. The noble retinue would start at full morale, the levy would start at the traditional low morale reflecting the need to bring the various troops together and to get them organised in some coherent fashion.
4 - Im very much in favour of naval modelling, and Id argue that coastal raiding/piracy ought to be organised along similar lines though there would be less direct noble involvement, and the impact would be more upon your non-proffessional merchants, reflecting their decision to engage in an aggressive trading strategy due to opportunity.
You could have several implications:
- Firstly, it gives everyone *something* to do. You can always get into the game by raising some of your noble retinue and risking a raid on locals you dont like - including rivals within your own kingdom. Means you dont have to sit around waiting for gold/prestige to clock up so you can do something, or waiting for a lucky event.
- Secondly, lot of potential for character interactions: raids could lead to capture of characters who would then be held in the enemy court until ransomed, friendships, rivalries, romeo & juliet style romances etc etc. Characters could even be the target of raids with the potential to embarass or humilitate people, or for elopement - Diarmait Mac Murchada, who brought Strongbow into Ireland, was already infamous for a raid on which he kidnapped a rivals wife (who wanted to leave her husband). To further insult his rival, he later sent her back.
- Thirdly, it has political implications: If a land is being constantly raided, the target has an incentive to try and find some sort of arrangement with his tormentor: either through the payment of tribute, or an attempt to become a vassal of the raider. Afterall, if the local noble cant protect his lands from raids then his own position is under threat as his own barons will seek an alternative ruler who can defend them. Again, in Gaelic Ireland, a Kings prestige and his ability to control his vassals was totally dependant on the threat of raids, which would impoverish and humiliate a weaker king until they agreed to become vasals.
Similarly - marraige alliances ought to discourage raids by one side of the family on the lands of their in-laws. A non-aggression pact of sorts. Perhaps incentivised through prestige penalties, which should be significant but not penal. This would give players another reason to look for a local, political marraige, rather than opening bride finder and sorting by Stewardship stat.
As part of this - raiding would be low intensity warfare, but if theres a constant wave of raids on a particular border, it should increasingly draw the attention of the Kings who might accelerate it to high intensity warfare. Especially if a highly influential noble is the target of the raids.
Raiding ought to be part of internal politics too: a King might ignore, or even encourage his vassals to raid the lands of a vassal he dislikes or who is too independant. The target of these raids should have only two options: seek to reconcile with the King somehow so that he no longer ignores the raids, or rejecting the King who has not protected him and risking the acceleration into high intensity warfare which might cost him his lands (which may have been the intention of the King all along).