Time in the game MUST run at real-time without pausing, like most MMO's. Why? Because if you're running a game and it's Granite 13, 2312 in one fortress, it can't be Hematite 24, 2300 in another just because the overseer had the game paused for 8 hours there, or Granite 1, 2993 for an adventurer who just started out. Also because the players are living in real-time, and want to be able to act in real-time. Everyone needs to be on the same page. This means that we can have 86400 ticks in a day, though, to be fair for adventurers and to give fortress mode players long breaks between possibly-debilitating events, to make up for the fact they can't pause it while they give orders. The way the game is now, creatures must wait a certain amount of time before making another action: with real-time, this will still be necessary so the AI isn't zipping around all over the place while players are mashing their keyboard frantically to keep pace. It means that those 86400 ticks will need to be divided into smaller units, most likely, and max FPS must be determined by a reasonable amount of time for humans to receive and respond to stimuli and for the server to receive that response and process it. Placing designations in fortress mode would be a breeze; the game will process your order in due time, and your lovable drunken morons will get it done as slowly as they can possibly go without getting magma'd, but adventurers making targeted attacks will make them look like an idiot standing around waiting to be whacked in the face by the auto-attacking AI. Not that taking the extra time couldn't be rewarding, but it may mean the game could only progress at 1 tick every 1-2 seconds, so things aren't happening too fast for players to interact with the UI (which would need an update as well) There will be many, many people playing all at once, so even localized time-fuckery probably couldn't be handled, and you can't have even an administrator handling pauses except on a schedule everyone understands (example: paused for 15 minutes every 45 minutes; a 1-hour cycle) or they'd all get pissy. Exactly what the difference between IRL-time and in-game time is at that point, I couldn't hazard a guess (because too lazy to math it, meh
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DF will be seeing a lot of "activating the world" going on over the next several months, and I suspect that it will ALL be absolutely necessary for this parallel-multiplayer idea to happen anyway. With everyone on the same page, caravans and armies can go about the world at whatever rate they're capable of, and arrive at their desitnations whenever they get there. Caravans could then leave if they're either dismissed or when the AI (or a player, when we can play as merchants) choose to do so. (the conditions the AI use to decide when to hit the road aren't necessarily important in this context; they can be decided later)
When a caravan leaves one location, it then makes way for another. The game WILL need caravans dedicated to moving between player fortresses, but it can also have many more caravans that follow whatever other procedures under whatever given conditions. This way, players can also trade with one-another, but having another overseer on a trade route decide to murder the merchants and take stuff meant for the next player in the route (by the AI or player-merchant not offering them for trade with a particular fortress) won't be taken kindly. Player-merchants may refuse to trade, even, if they don't trust an overseer or the "defenses" around the market/depot, which may mark the end of the trade-depot drowning chamber.
The travel map for adventurers, though, would essentially be reserved for use as a navigation aid; you could only really travel in real-time, anyway, because everything happens real-time. Unless it can be used to tell the game "I want to go to this location, I'm gonna be AFK while YOU take me there. Use roads at your discretion." The AI auto-pilot could handle hostiles if you're actually AFK, or you could of course take control back. Players would probably use this auto-pilot a lot, because they wouldn't want to sit at the computer for the 8 hours it may take to travel 10 miles in-game, and logging off the server couldn't realistically pop your adventurer out of existence like it does in many MMOs; you'd still have to persist, for the sake of your companions and auto-piloting. The AI could attempt to hide and sneak around, of course. Another option is teleportation magic being made accessible to adventurers*, and they could instantly travel anywhere marked for teleportation that is either publicly-accessible (cities and towns, and player fortresses where the overseer(s) permit) or part of their own private system.
*The cost of such powerful magics would inevitably be high, for balance purposes. A "cooldown" is reasonable, as well as a prerequisite ritual (maybe with certain reagents) for the spell that involves summoning yourself remotely, or something, and a long period of concentration to accomplish it.