The first question I'm not so sure. From what I understand at some point you will be able to send armies to other places on the map, although not control them directly. (You would remain playing on the fort while the army is away)
I may be totally wrong.
There are also mods (Masterwork comes to mind) that adds this sort of thing, within the game's current limitations.
Other question is about the world. You can create a huge world but your fortress is just at one point so
I would like to know why do you need to create such big world with al these events and history?
For starters, if you play on adventure mode you can travel all of that world, even visit fortresses you've played with long after they have fallen, and actually battle hand to hand with whatever destroyed your fortress.
Regarding fortress mode, apart from giving you options of where to settle, the world generation defines the history of civilizations, who was born and how they lived their lives, affecting the skills, age, genetics, perks and loads more of the migrants that go live into your fortress. You can even recieve a former adventurer you played with, as a migrant to your fortress. (That's what I've heard, I haven't played much on adventure mode to try this)
On the next update the world that you generate will continue living while you play on your fortress, and the events of the world will interact a lot more on what happens on your fortress.