What’s missing is a challenge and accept system. Back in the old days, there was a game where you placed down a challenge on an enemy fortress (in this case it could just be the entire clan) and they had 2-3 days to accept and set the date (which had to be within 2-3 days of accepting the challenge). If they didn’t accept then the default date set by the challenger was set and on that day during a window (for NA servers it would like 5pm-10pm) their buildings were completely vulnerable. For example in Conan Exiles -
Clan A sets down challenge banner in the building sphere of any of Clan B’s buildings. The current day is Monday, they set the default Siege time for Wednesday and the window is preset by the server (NA severs 5-10pm).
Clan B logs in Tuesday and sees they have been challenged and sees they have the option of setting the siege for Wednesday, Thursday or Friday. They choose Friday and Clan A gets a notification about the siege.
Clan A now has to wait, but with the upcoming alliance system they can speak to other clans and make allies and recruit anyone on the server temporarily for the scheduled friday siege, but so can clan B.
Those alliances might decide who gets sieged next. They can place trebuchets on the outskirts of their foes base too.
Anyways, Clan B now has to defend their treasure room for 5 hours, losing all their treasure might result in something catastrophic like their whole structure losing stability. (Every connected piece).
Regardless of the outcome, Clan B becomes challenge immune for 3-5 days.
This might be more of PVE-C type concept, but this would guarantee no offline raiding and would build up the hype and drama of a siege.
And the game was called ShadowBane and it was beautiful.