-There are two things that I think will either make or break this feature. One is that units must be able to retreat from the area after raiding, and the second is that if your buildings are raided they will not be completely destroyed.

Retreat after raid: The reason for this is that if they are not allowed to retreat afterwards(Given that they have the movement points available) then the only viable way of raiding is with armies comparable to the defending army, in which case beseiging the province main is a far superior alternative. What would be best is if raiding time is related to army size so that you don't have individual units cheesing around all over the provinces. If it's just one unit then it will take far longer than your turn and so you won't be able to retreat again afterwards. Various unit types could of course more more skilled and require less manpower and so on.

Building destruction: I'm sure as hell not going to keep rebuilding every time the AI stumbles across my border to perform some inane suicide tactic. It's not "increasing difficulty" or "enhancing realism or gameplay", it's just annoying and boring. I spend enough time performing the same tasks in every damn province as it is and this is just an annoyance that would kill gameplay. The best option would be automatic repair which would take a shorter amount of time that rebuilding from scratch, with some manual control over it as it would have to drain your resources and I would not like to leave that up to the AI. Alternatively you could have several options when raiding, from simple drive-by pillaging to large scale decimation of the countryside, each option having an increased duration to complete, relative to army size.