Nidbanes are pretty useless right now and have already lost their original use: being a method for new players to "get back" at people who harrass them.
While I know siege is next to impossible currently, I think that if there are more ways to provide defenses for a village, that the current restrictions on the siege system could be uplifted as players gain more and more options to defend themselves- so, take the below with an assumption that if it's implemented, there would be some sort of look at the offensive side of sieging to make it more interactive/easier.
As for the suggestion:
Nidbanes, or an alternative form of nidbanes, should instead act as protectors of a claim rather than spirits used to enact vengeance and justice. They should be created in the same method (create a nidbane fetter, and place it upon the runed dolmen), but without requiring a scent. Nidbanes without a scent instead roam around periodically in the claim it is created in; If there's a village claim, nidbanes should be a bit weaker than if there is no village claim. There should be a maximum amount of nidbanes that can exist on any claim.
Nidbanes should periodically roam around, and 'protect' a village from would-be criminals. Each nidbane should have a duration based off the quality of the fetter, and stats based off the stats of the creator of the nidbane. When someone trespasses on a property, nidbanes should swarm the hearthling, but not attack- if they create any rummaging, theft, assault or vandalism the nidbanes should immediately begin an attack on the perp.
Nidbanes can be slain, but rather than shattering the fetter, it should 'deactivate' the fetter and cut it's quality down (thus reducing the maximum duration), until the fetter is placed again. You should also be able to 'feed' a fetter scents belonging to a particular person, granting that nidbane higher and higher stats if the person who created the scent has higher stats, again weighted by the severity of the offense. Feeding a nidbane many different heinous scents would cause the nidbane to 'break free' of merely protecting the claim, and act much as they do now- powerful, singular beasts that chase down the person who committed so many different crimes against a single village. The nidbane should be untargetable against anyone but the perp, although only one such nidbane should be able to hunt a person down at a time.
For village claims: Nidbanes should become inactive if too many characters oathed to that village are currently online; essentially meaning they are mostly guardians of a place whenever people in that village are offline, or for hermits.
Essentially, Nidbanes become protectors of villages, and if a particular person continually harrasses and commits crimes against one village, they would turn into much as they are now, except more powerful and less spammy.