A. The event involves a series of stages and different types of FATEs, beginning with some slow starter FATEs so people can know that it's happening and be prepared to take part if they're unavailable at the first few (so like calming citizens, putting out fires, fighting looters in the street - random events in X_City). Assume that between stages, and specific types of FATEs, certain amount of time (the whole thing is not going down in 10 minutes, but over a few days).
For lower level DoW/M there can be inside the fortified area FATEs, defending lower level enemies, helping control the insides, bringing powerful tonics to players and notoriary NPCs alike (give Raubhan a drink :3), and also consider there may be a Favor of Light buff that makes enemies deal less and take more from these lower level players (not enough to make being higher level not as advantageous, but enough that lower level players can take part too - more defense buff than offense).
B. Building up with escorting citizens and maybe important NPC to places to prayer (to power up the warriors/mother crystal/prayer shield), when these escort fates take place the air is filled with a thick black miasma and players have to protect not only citizens but these protective magitek that create domes of light, mini prayer shields (protection against miasma and void monsters teleporting in). If a prayer shield breaks down crafters need to repair it while other players defend them (interact with the device, opens up a crafting menu / no material cost to player, success increases the life of the shield - also to note the shields should be on carts that crafters could ride and preemptively craft as well as feed boosts to the shield that grants boosts to those under the dome), the darkness creeping in at broken points - shout emote at NPC will get them to unfear and move into nearby activated shields.
C. Events for gathers will appear as well once a base is located where players can protect other gathers while they attempt to obtain resources for the strongholds being made near prayer sites, the success of these helping later stages - the player getting the material from stone, tree, and sea and then players defending gathers who will have a marker when their full on the unique materials as they run to the drop off location. The number of different locations will help separate players from crashing zones / needing many instances of said zone, how players are divided could be done through the GC or some other system - like talking to an NPC General on where they think you should go. There is some leeway in this, just considering that a server wide event will need to have systems in place (whether literal, or through gameplay) that ensures that the whole thing doesn't turn into a crash.
D. Through these events the prayer sites becomes fortified over time and a few assaults happen, like as the through the gathering moments, have cannons and other tools players can use (another tool for lower level players to make a greater impact), waves of enemies (some big waves of fodder, others mini-odin like fights). Aht Urghan sort of feeling (forgot the name of the content in FFXI lol). Important to protect certain NPC to make defending it easier later. After the set of growing difficulty waves over a period of time a status report is made available to players on which areas have what defenses so players can plan accordingly. 20 Minutes before the final massive FATE wave the report of each areas defense also includes player count, so people can organize better (okay this area is low defense and has 12 players, we need more over there!). In between and before events having occasional crafters FATEs to help bolster lost defenses while other FATE events to obtain NPCs who were abducted, including trying to sabotage areas inside the void and get out ASAP. To note while higher level may mean more points it should let anyone gather/craft, the materials needed being free and the difficulty of the craft scaling to the player (but again being high level would be better but not required) - perhaps to prevent a bunch of people messing up on it, after a set amount of failed attempts (like 10, one of the NPC generals suggests you to join the fray as a DoW/M).
During the last big fat push, filled with waves of varying types of monsters (certain sets have certain mechanics, like their ability mechanics, types of aoes, death runners, how melee and ranged behave, what tanks should pick up), some players would be marked with a holy light and they'd have to go into the back and pray. If enough prayer is collected in one area and another falls, that fallen area can be revived with temporary sanctuary (pushed enemies out). So like Horizon could make a last stand and save Forgotten Springs, when one group succeeds they can teleport into other on going sieges.
While there would be points for failure that don't lead in the entire event's failure at points where logic dictates the whole mission/event is now fully failed players would go black screen hear a ticking clock sound and wake up abruptly in their beds, with the FATE happening again sometime in the near future. Although ideally, people die - a lot, areas fall and rise, but so long as the whole server works together the FATE series will be won.
E. Once the prayer stations have been loaded up enough the city in question will begin to power their prayer shield while the entire city becomes a battlefield, optionally those who want to enter the regular city can do so by selecting an option at the zone "remember a better time". The battlezone FATE victory being a tug of war determination, gain enough points win to the next stage, lose enough fall down a stage or to entire mission defeat. The last stage would be the shield shattering, leaving a net that slows the meteor but is no longer holding it, with enough energy remaining that the warriors of light enter into the device and get launched out like a weaponized aetheryte teleport (bolt of warrior lightning lol) - landing on the meteor's surface where they fight a giant FATE boss with a few stages in a fairly large meteor zone (be mindful that there may need to be "craters" to represent different instances so the whole thing doesn't crash due to amount of players). Crafters can help build small healing stations and other useful to player outposts on the meteor (that's falling slowly towards the city, so if you look up there's the city lol), while gathers up to a specific point can find unique resources which will increase the reward of the FATE once won by the DoW/DoM (and or improve crafters results, so things get built faster/better). If they can get some Chrono Trigger sort of Lavos theme with the boss, and the meteor, that'd be perfect lol - including the idea of multiple stages like with Lavos.
There can be a few stages in the boss fight that encourage some time gap as well, so that other players with different timezones can take part more easily; however, if you've taken considerable part through the series you'll get the major rewards at the end regardless if you were there for the final blow (but it'd be cool and a nostalgic moment to be there at the end). If there is a cutscene, those who took enough part can "recall" it later if they missed the even for whatever reason, no need to let them feel worse (not getting to be there with everyone is already enough of a downer). The time frame for the final stages would be consistent/known enough that players once they know that part is happening could plan for it, like SE announces that part VII of the FATE is two hours long and will start three hours after VII (so you can take time off work LOL, or whatever they need to do - tell their SO dinner is take out :P).
Perhaps the event revolves around the anniversary / bard. Happens once a year, maybe not on a specific day but a specific time frame.