It's pretty much universally agreed that it's lame how people can undo all your work while you sleep, and claims, walls and other existing protection measures don't do much to stop them. To address that, I suggest making an in-game distinction between thieving raids and destructive raids.
TL;DR version:
- it should be possible to sneak into a town and steal stuff while the inhabitants are logged off
- it shouldn't be possible to sneak into a town and destroy stuff, chop trees and slay cattle while the inhabitants are logged off
The long version:
Theft and destruction are two very different things, yet they rely on the exact same mechanics (wall-bashing). People can cope with theft, but thorough destruction of one's place is enough to make most people give up on the game. In the current system, any nerfs on destruction (such as raising the strength requirements for wall-bashing even further) will also nerf theft, and IMHO a bit of theft adds some spice to the game. That, and thieves relying on strength seems just plain wrong to me. So, here's suggestions for two replacements for wall-bashing, one for thieves and the other for invaders.
The invaders should have to breach the walls before they can go in and wreck the place. The defenders should have time to react. This is easily achieved by requiring the invaders to construct siege weapons, ranging from rams to trebuchets. Let's admit it, smashing walls by hand is a bit silly anyway. Anyway, these siege weapons would be built by constructing a siege tent (a la Stronghold) and waiting for the weapon inside to "mature". Simpler weapons take less time, so a ram could be done in twelve RL hours whereas a trebuchet might require two days. Palisades can be knocked down with rams. Brick walls need catapults for the wall itself, but the gates can be brought down by a ram. Castle walls (new wall type) need trebuchet for wall, but the gates can be brought down with a catapult. Fortified castle walls would need a trebuchet for both the walls and gates. The general idea is that the more you invest in defences, the more time you get to react to an invasion. The siege weapons should be constructed on site to ensure that the defenders are warned, so something like maximum distance they can travel is in order (and some, like trebuchets, should be immobile).
Once the walls are breached, the invaders would drain the authority of the village by destroying authority objects or something like that. It should take time to drain it, so that the defenders get one last chance to repel the invaders. Once the authority reaches zero, the village is disbanded, and the invaders can relight the idol to take the village for themselves.
As for thieves, they should be able to simply go over the walls with a grappling hook. Climbing would require an agility check, and in case of failure the climber falls and takes some damage. To make sure they can just thieve, it shouldn't be possible to scale walls while wearing armour or carrying (in inventory or equipped) any items that take multiple slots. Clothes should be an exception, so that you can wear a cape while you steal. Anyway, destroying stuff should require a pick, and killing cattle should require some multi-slot item. Furthermore, crafting should be disabled on claims where you're not allowed, so that a thief can't bring materials with him and make a pick on-site. If you leave a pick in an accessible place, well, sucks to be you. Finally, it should be impossible to teleport while you are on claims where you haven't been granted access.
Since walls no longer offer protection, locks are in order. A padlock item would be nice, one that can be put on containers and doors. The thief would need lockpicks to get past the lock. A dexterity check that also takes into account lock and lockpick quality would work well, I think. On a failed check, the lockpick should break. This should somewhat limit the damage, as getting in and out repeatedly would take time and hp (there should always be a chance of failing the climb check) and you can only fit so many lockpicks in your inventory. That, and lockpicks themselves should be fairly expensive to avoid lockpicking spam.
In addition to thieving, this could also be used for scent assassination. A single slot dagger would be nice to have for that. Additionally, sabotage of siege weapons should be possible. Sabotage wouldn't outright destroy a siege weapon, but it would render it inoperable for a set period of time, or lengthen it's construction time if it's still in the tent stage. This is needed so you can build your own fortifications (palisade, most likely) around your siege camp when you're trying to invade a village. That, and so that the defenders can sneak into the invaders' camp and sabotage their siege tents to buy some time.
All this sabotaging could lead to unending sieges, as neither side manages to get a functional siege weapon, so additional defensive measures are in order. Namely, guard dogs. A dog shouldn't be much of a threat to an armed and armoured character, but getting attacked by multiple dogs should be seriously bad news for an unarmoured thief/assassin/saboteur. Dogs should deal a lot of HHP damage on knockout and be faster than hearthlings, so it's possible to get killed by them before you reach safety. It should be possible to temporarily pacify a dog by throwing it a piece of meat, but that should just give you enough time to run away. Stealth should help against getting detected by them. Dogs should have a significant upkeep, so that one can't cover his whole town with dogs and effectively make it invulnerable to thieves.
Whew, that took longer than I expected, and I still have a feeling I forgot half of what I meant to say. Anyway, I apologize for the wall of text. If you actually read it, you'll notice it's mostly analysis, and I tried to avoid any new items that aren't in the devs' plans already, but some (such as grappling hooks) were inevitable. Suggestions on how to further reduce the number of new features required for this are very much welcome.