It is interesting to see where you are taking this, but that is counter-productive to what i meant by initial post.
Building analogies and scenarios in the shape of "what issues can come up if DF would be like WoW..." is not what i wad suggesting. I explicitly try to avoid these areas and stay in the realm of possible. Of course pausing in real time strategy game when i comes to MP is not acceptable, of course adventurer mode in MP will suffer from many logical and technological issues, of course roaming as adventurer into someone's construction site does not really make sense and introduces whole bunch of issues stating with "why?"...
Yet it still feasible and possible for fortresses to coexist in same universe and share information and some items. It is also possible to explore that world as a single player adventure, find forgotten and abandoned fortresses and meet unforeseen challenges. And it i lso possible to create history and legends of that world all together w/o needing to turn this int full blown MMO.
This game was never meant to evolve this way, and i don't see why Toady would want to consider this angle... but certain aspects of in-game community can become greatly beneficial and enhance the game play, without turning it into another fantasy MMO game.
so i'll try to stop this by answering a few:
Imagine two players meeting under adventure mode...
no, they don't ever meet, adventure games like rogue are single player experience. DF is not going to become what MUDs once were.
... and under Fortress mode though nominally 'realtime', what happens when Player A advances upon Player-B's fortress
no one advances anywhere, it is a settlement management game, you deal with your dwarfs and he deals with his
I think the multiplayer system comes up against the barrier of the 'ticks' for each player going at different speeds....
combat mode would take place somewhere unrelated to player's settlements and should only be about squad management and tactics, amount of activity will be severely cut down and the situation should not even approach typical DF resource consumption. That being said synchronization amongst players is still to be achieved and there are different methods for doing so, including very low yield ones as far as coding and maintenance goes.
... if I'm playing the dig deeper mod and try to trade a blackmetal chestplate to a player who is running vanilla, his version of DF won't know what to do as neither blackmetal nor chestplates are in the vanilla raws.
Some object compatibility can be granted within proposed trading mechanism, where objects can be interchanged based on functionality, but aside of that - what XVIII century engineer would do given a '69 mustang carburettor... or modern average John Doe with clepsydra? If it appears to be "an alien object of indescribable shape made from unknown material" and can be only used as fancy looking paperweight within your civilization - may be you are better of bartering something else that you find useful? I remind that trading service as I described it only meant to expand existing functionality and create additional options.