I will be very disappointed if SE drops the ball on this one. Campaign/Besieged was one of my favorite things to do in FFXI simply because it was a really fun way to kill time. The fighting itself was fun, and the rewards made it actually feel rewarding on top of that.
If they pulled off both of those factors, I think it would boost the longevity of patch 1.21 and proceeding patches by a massive amount all the way up to 2.0. The problem we have right now is after a patch drops, people are left with not much to do once it's all completed. Where as with the potential Hamlet Defense has, we would all have something to do to kill time and have fun, all while being rewarded for our actions.
I really, really like the idea of building up hamlets. That seems like the absolute perfect idea to make an involved experience for everyone. Disciples of the Land could get quests to supply material, and Disciples of the Hand could get "leves" to make material to fortify the hamlets out of the resources brought in by disciples of the land.
Gates could be fortified, buildings could be fortified, NPC garrisons could be outfitted in stronger gear and better weapons. Canons could be crafted as well to help in the defense.
And as noted in another post, the more fortified a Hamlet becomes, the stronger and more frequent the Imperial (I'm assuming this is going to be the Imperial V. Eorzean conflict) forces would become. It could just start with footsoldiers and their war hounds. Eventually you'd start seeing centurions mixed into the equation. Eventually the footsoldiers could be phased out in exchange for the empire's elite centurions. Perhaps even have Juggernauts appear at high enough fortification levels.
Perhaps if players reached a "maximum" fortification level (not something that should be easily accomplished), a Dreadnought could appear in subsequent raids and rain down fire from above using a targeting system similar to what the Juggernauts currently use in-game to add yet another level to the chaos.