Posting to watch... very closely with interest.
Yes, these ideas are good for after the basic implementation is working more solidly. The code so far is not too complicated, so it should hopefully be quite adaptable by anyone who wants to build a mod ontop of it. Regarding PvP specifically, I'm sceptical because I suspect there is a lot to untangle -- is it possible to get a squad from one side to target a squad from the other? Can we prevent dwarves from becoming friends with each other across the gap? In comparison, co-op seems like a natural extension, at least from the technical side of things.
Also better late than never, fundamentally because they're army-controllers it should work exactly the same, though there's nothing for sure that the eventual result of watching your dwarves leave the fortress will definitely translate into them arriving intact to siege the player, and that they don't just end up being a fully equipped migrant wave in the confusion of what 'raid' means when not handled by the AI.
If the Adams brothers ever back to support issues around this multiplayer specific problems of interactability or even to officially support a standardized multiplayer mode in the future it'd probably be good to have some kind of unique tag used on the mantis tracker.
For all we know if the technical limitation of having two geographically distinct forts is possible, possibly even in different dwarven civilizations as to provoke different cultural conflict (via tab switching civ, choice being dependent on what modifications are present if you wanted to grease up your goblin mass army skills against a dwarf player) you could set up army controllers to go places, or when implemented intrigue people to arrive, steal & coerce (like visitors do at the moment).
Any of you folks ever play the original startopia PC game? Seeing this close together multiplayer is bringing all sorts of old RTS nostalgia back to me of a couple of people living virtually on-top of each other occasionally struggling over workers and space to run efficiently.