In the recent dev update, Drew noted that there's a lot of community speculation that diving and hourglass are the reasons that there's less and less encounters on the seas and the feeling of empty horizons. Drew then goes on to note that they've discovered a "smoking gun" associated to a server migration tech "problem." Obviously, he didn't get get into specifics, but it sounds like they're acknowledging that server migrations are a problem, but are not willing to admit that diving, in general, is the true underlying problem.
Diving IS the problem. You wouldn't have to do all those server migrations and have fancy bandaid tech to try to keep servers populated and full of life if you didn't have so many crews diving in and out of servers. Further, specific to hourglass, those battles consume the life of 1/3 of a server's population for 'X' amount of time before they inevitably need to be replaced with other forms of life.
Don't overthink it, Rare. It's Diving. Raids, voyages, hourglass, all of it. Diving IS the smoking gun.
Solution: Just some spitball ideas, but here goes...
For Hourglass, this should be obvious; that can be made into a main menu decision, and all those battles can be on their own servers with whatever level of event/emergent toggling you want to do (like you have with Safer Seas). Unlike Arena, they would not need to have a separate scoring system or menus or anything that requires its own unique maintenance which caused Arena to be a resource hog and subsequently played a part in its demise. I'm extremely hopeful that the stuff next season proves as a testing ground for some things that lead to this.
For Adventure, this is going to be a multi-prong solution.
-Voyages. This one should be obvious; just like, go back to the old voyage system or heck even the captaincy voyage system would be better. The "fast travel" for voyages is saving, at most, like 3 minutes of sailing (and that's being optimistic). You're not improving the pace of play by a worthwhile amount at all. Plus, most people fire up SOT expecting it to be a longer play session, so there's little point in the miniscule amount of time savings.
-For Raid Voyages on the World Event "Lite" encounters (Sea Forts, Shrines, Treasuries, Skeleton Camps). Again, those should be obvious; lay down the raid voyage and it can choose either the nearest one (or nearest one without another crew in the area), flip the loot to whatever company your raid voyage is for and boom done. Easy.
-For Raid Voyages on true World Events, these are a bit more complex. Honestly, I'd just get rid of them, but I doubt Rare would consider that. Instead, they can operate in a queue fashion if an event is currently active and a crew is doing the active event. Or, if no crew is doing the active world event, then just like today with these Raids, it will de-spawn the active event and spawn whichever one you are looking to do. If you throw down a Raid, you (or some other crew) have 15 mins to get to it and start engaging before the "voyage" is automatically failed/cancelled.
"But what if I was doing the active event, sank, and want to go back. Will someone's Raid overwrite that?" That 15 minute timer starts anytime you're not in the radius of the event. Get back in time and you can keep going...or it'll cancel/despawn if you can't. These should have a similar mechanic to the Skull of Siren Song in that you get a close enough respawn that there should be no excuses for not being able to get back.
"But what if I'm on my way to an event and someone drops a Raid before I can get to it?" Tough cookies. That's no different than today where an event despawns while you're in route. It happens. Get Rared.
"But what if another crew has a Raid queued before mine?" Well, that's why it's called a queue. Wait your turn. Go contest that event. Go help them complete it. Switch Servers. Whatever.
Again, I'd much rather these go away altogether and those top tier Raid loot items that drop in them be put into some other voyage or something instead.
