No, this thread has nothing to do with NPCs, despite the mention of guards.
The Summoned Strike Back.
Alas, poor Loftar may have to code some AI if any of this is to be implemented. Basically, summoned players should fight back. This was once theorized around the time summoning was first implemented, but nothing ever became of it (that I know of). I think it would help the current state of PvP quite a lot.
New thoughts on this: The Summoned should have a higher or lower percentage of their offense and defense bars when they are summoned, depending on their exploration and perception in comparison to the Summoner's stealth.
Obviously, the hearthfire itself would have to be able to path to the Summoner before he could summon the Soon-To-Be-Summoned. Lest the Summoned be potentially incapable of defending himself, from inside his own walls, for instance. If ever the Summoned is unable to path to the Summoner, the Summoned should be immediately un-summoned. The Ex-summoned should probably be safe from summoning for a while, to prevent abuse.
"Protection," and "Protectors."
A simply game mechanic. In the Kinlist, you would have the option to "protect" another player. Some icons would probably help, maybe a pair of crossed swords if you were protecting them, and a shield if they were protecting you. Any number of protectors should be possible.
Explanation: In the event that Player 1 is summoned by Player 2, Player 3, who is protecting Player 1, has a chance of being summoned with Player 1, to aid in their defense. Whether or not Player 3 comes to the aid of Player 1 depends on his/her exploration, perception, and the distance between their hearthfires (less is better), these things are calculated against Player 2's stealth.
If ever the summons were unable to path to the summoner, all summons would be immediately un-summoned, et cetera, et cetera.
The Night Watch
For villages. The Lawspeaker and Chieftain would have the option to add members of their village to the Town Guard (or some mythological name, such things are superfluous). The Town Guard would act as protectors to all the village members. Whether they come to the aid of the Summoned is a functionally identical calculation to the normal protection mechanic, with one exception. Instead of calculating the distance between the Guard's hearth and the Summoned's, the calculation would depend on the distance between the Summoned's hearth and the nearest village authority object.