I get the sense that the main way dwarf fortress is played is: Create a world. Play a Fort. Create a world. Play a Fort. Etc.
I have been no different in that regard until I started playing the world I'm currently playing.
One observation I have is that player forts are extremely attractive to all the forgotten beasts. So much so that my world has actually run out of them over the course of just two forts. There are exactly 3 FBs left in the world.
There is one living in the caverns of my first fort. I don't remember the reason why, but I suspect that with the Low FPS I had at the end of that fort, I just didn't feel like dealing with him before I retired.
I have one living in the caverns of this fort as well. He's currently stuck in a small patch and can't get out. I'll probably eventually figure out a way to dispose of him.
The last one is living in the ruins of a fort he destroyed during world gen and has shown no inclination to ever leave.
A couple of thoughts occur to me.
First, I suspect that the utility of the caverns is likely greatly improved now that I don't have to worry about FBs just randomly wandering in. I would appreciate any thoughts from overseers that have been in that situation before!
Second, FBs really need some sort of tuning to be viable in the more long term. Between just 2 forts, I have had over a hundred FB attacks and they pretty much all end in one of 3 ways.
1: The FB wanders around killing things, leaving behind a huge FPS killing mess until it eventually takes on one too many crundles/elk birds/whatevers and the lucky hits start to pile up and he dies.
2: The FB wanders around killing things, leaving behind a huge FPS killing mess until it eventually runs into another FB and he dies.
3: The FB wanders around killing things, leaving behind a huge FPS killing mess until it eventually gets stuck between/in some trees and I send a group of marksdwarves to shoot him.