Dream Server Concept (Expanded):
The more I thought about this the more I realized I did a poor job of fully outlining the idea presented in my post above. As stated above, a modular overarching plot that drives the server for years. Ideally one open ended enough that it could be supported forever (who thought we would still be playing on this server 10+ years ago ....). Some ideas to accomplish this, in my mind, are listed below.
(1) Theme. Come up with an over-arching theme / story. The coming a majorearth shattering event, rise of an empire, etc. Determine multiple major factions that would be working for and against this event. Ideally these factions would be struggling with each other as much as they would against their opposing "side".
Basic example, a volcano is about to erupt and the local druids solution is to conduct a ritual using a willing human sacrifice to stop the event, local mages want to build a magical shield to funnel the explosion into the sea, and the lazy mayor of the local town wants to keep it all quiet because he thinks the signs are fake and people are panicking. Ideally the steps needed for each faction to "win" cause each faction to work against each other even though they are all on side "survive the volcano".
The key here is that these major power conflicts happen OFF SCREEN. In our basic example the players would be some small hamlet located near the local town but not part of it. The players would be recruited to support our three NPC factions to help that NPC faction's cause. The idea is that the main NPC factions are evenly matched and so they need the players help (much much weaker) to try and tip the scales. Since most of the conflict between the factions happens OFF SCREEN, players deaths / players leaving / etc doesnt kill the major over-all plot arc.
Last part on this, ideally some of the major factions that are working against the volcano (in this example) would NOT like the player's hamlet. It gives the DMs multiple "big bads" to force players to work with / work against while at the same time still having to deal with the main over-arching issue.
(2) Modular Chapters. Each chapter has a certain point and last a good long time (years+). In our basic example the point of the first chapter might be to solve the mystery of if the volcano is actually going to explode at some point. Ideally breadcrumbs that there may be an issue would be placed easily accessible to players on log in, but finding out more would require significant effort. This effort would likely entail exploration and IG mechanics (lore checks, questing, exploring, talking to faction PCs, etc). The idea is to give all PCs some basic thing that PC can always be working towards during that chapter. Something that all PCs would likely want, is very very hard to obtain, and can be approached from a lot of different angles. Good idea is to have the final couple of keys to finding said event be something DMs must support.
(3) Start Date and Mystery. When you come up with the plot arc start the server in the middle of the plot arc. A good example of this is Malazan Book of the Fallen series. Doing this lets you do two major things. First, it provides mystery as you aim NOT to tell players all the details of what has been going and what the main issue is. Give them just enough to details to have a rough idea and leave as much of it as possible to FOIG mystery. In my opinion its story that truly drives servers like CoA, give them a story to unravel and a mystery to solve. This also lets you tie in relics / powerful artifacts / powerful abilities that were crafted by the main NPC factions in times past and now can potentially make their ways into player hands. With our example above, you start out as a new arrival in the hamlet and hear some wild rumors about the mountain.
(4) Player Factions. The player factions would ideally be smaller factions that serve a purpose in the player settlement(s) and can potentially support or work against the moving / shaking NPC factions driving the story arc. With our example; hamlet town guards / local ranger wildlife group / merchants bringing in goods from the outside world / etc. Idea is that these factions can be kept unimportant enough to rise and fall without causing major disruption to the over-all arc of the sever.
(5) "Little" NPCs & Consequences. The goal with all of this is to continually provide smaller NPCs players can work against / for that is advancing the over-all plot of the server that is every so slowly being revealed. Ideally the actions for / against these NPCs would have consequences that fundamentally change the server. This lets you keep the server fresh (new player factions, settings, etc) while keeping the same overall plot arc. Key part of this is actions and consequences, the LG solution to everything should not be the "right" way all the time. In our example, oh you killed the horrible necromancer with a crypt deep in the volcano ... great he isn't raiding the hamlet to gain victims for his vile rituals ..... he also isn't keeping up his ward he was powering to try and keep the volcano quiet .... err doom is now more likely.