OK, well, I think I have how I want to articulate this, now...
To start with a metaphor, I remember that the game Cities XL was originally meant to be a sort of MMO SimCity, but the company couldn't put together the money or servers for an MMO, and went bankrupt, so the company that bought the game just sold it Singleplayer. The game was originally meant to have each city require resources that it couldn't produce, itself, so you had to set up trade deals with other player's cities. In the released game, however, when your main city needs something, you have to save it, then start your own new city to generate and trade away that resource you need, and start trade between your two cities.
Much of the original idea behind Dwarf Fortress, as I remember it, was about how it would be cool to let players build a fortress, then, when they are done with it, they could revisit that fortress as an adventurer or something, and see the impact that fortress had on the surroundings. Toady's talked about how he likes doing things like minting coins, then seeing how far abroad those coins can be found in other markets. The whole "Losing is Fun" thing started, at least allegedly, as a way to encourage players to keep playing the same world over and over, because every continuation of play in the same world compounded the history and meaning of that world to the player.
Now, with retirement and caravans/taverns that allow for player fortresses to interact without crumbling, I'm wondering at the direction Toady wants to take with encouraging players to reuse the same worlds, building more and more forts that can interact.
Since I remember a large portion of why mineral scarcity was introduced was to accomplish something similar to this for the caravan arc down the road, where you need more constant trade between forts, I'm wondering if there's something similar to what happened in Cities XL planned for DF.
Furthermore, the only "multiplayer" DF has ever had has always been from "succession games", where saves were passed around. Older suggestions at multiplayer were based around an idea that multiple forts could be played in the same year, and then at the turn of Spring, a save merging could take place to allow "simultaneous play". This relied upon the continuation of world events largely only happening on an annual basis, and determined at year start, however. The (relatively) recent world activation, however, shows that the world is to be running more-or-less constantly in the background, which scuttles any attempt at mock-simultaneous play without using some arbitrary limiters of how worldmap actions take place.
Toady, since we now have both retiring forts and you are adding in more free-wheeling caravans, traders, wanderers and hill-dwarves, plus there are going to be starting scenarios, I'm curious about how you want retired player forts to interact with active player forts in the long run. Are you going to try to get towards a retirement system where forts have saved import needs/export capacities or other pushes/pulls upon new player forts?
(I.E. build a new fort to support the next fort or sustain your last fort. "The mountainhomes need steel, set up a mine, and export iron or steel." "The mountainhomes is rich in steel, now we need you to build a watchpost at this location and train these steel-clad but rookie soldiers into stalwart defenders of the realm.")
Do you consider potential goals like potential "mock-simultaneous" play or taking steps to encourage succession multiplayer across multiple forts (sharing world saves after abandoning/retiring) as a means of encouraging play in a shared history worthwhile? For that matter, how much is this notion of compounding historic depth still a driver of your focus as a developer after all this time?
EDIT: Correcting typo