In pocket worlds everyone can reach everyone, so conflict is common.
If you have megabeasts, semi-megabeasts, and titans, they will of course do what they do best.
If you have demons, they will lead goblin civs, to great effect. If you permit civs to have more than 10 sites, goblins will often overrun the world.
To counter that, you can increase the number of civs, but lower the number of sites. Increasing the number of sites and lowering the number of civs will also work, it'll just make goblins stupidly powerful and high population.
I've found that a world without goblins runs incredibly fast, compared to one without. It may be that the background simulation takes quite a lot of processing.
These days, I ask myself why I want the other civs? Is it for trading, or some other interaction?
In fortress mode, your interaction with other civs is limited to caravans, sieges, skulkers/thieves/kidnappers, and liaisons. If the goal is danger, you can remove goblins entirely, and crank up the megabeasts, semi-megabeasts, and titans. However, this will lead to civ death in a big way.
Necromancers provide more danger, and don't remove civs (or at least, not as much), and are hard NOT to get, in the right kind of worldgen.
Similarly, removing all the megabeasts, semi-megabeasts, and titans, necromancers, vampires, werecreatures, etc, and just having goblins will lead to massive sieges very quickly provided you hit the pop cap of 80 within a few years.
If you don't want to wait that long, you can set the megabeasts, semi-megabeasts, and titans trigger to 7, and get them attacking in the first year.
Worlds with a tremendous number of unique underground regions will have a tremendous number of forgotten beasts, which, barring all other kinds of fun, are always present, even if you remove demons, goblins, and everything else 'dangerous' as vanilla dwarves require at least one cavern layer.
Finally, more often than not, these days two things happen early in worldgen. The first fort is overtaken by a megabeast, semi-megabeast, titan, or forgotten beast, and the second is most forts are, for lack of a better phrase, not purely dwarven. They often have everything from gorlaks to raven men to humans, goblins, and elves in them, and often MORE of them than dwarves, in some fortresses.