I did a quick search beforehand but might have missed something, so please tell me if this has already been suggested.
In Dwarf Fortress I'm sort of torn between what length of history I should pick during worldgen. On one hand, a shorter time means that there's still lots of cool creatures wandering around. On the other hand, you can only really get sprawling civilizations if you let it run a little longer. But it seems it's hard to get a world with a nice population and lots of towns, but also tons of cool creatures such as titans and dragons in the world. Of course, Megabeasts would stand in the way of the growth of civilizations and could even wipe them out, but what if they got a bit smarter and stuck together instead of wandering around wreaking havoc?
Which brings me to my idea: Megabeast Civilizations. Now they wouldn't be civilized as, say, humans would be. There wouldn't be any towns or anything like that. But it means that creatures like dragons, rocs, titans that have traits in common and other megabeasts that are able to reproduce will try to seek each other out, claim a territory and shape it into their home. I imagine it would be a concentrated amount of larger-than-normal lairs that they use to breed and hoard items. Creatures such as Rocs, Dragons and aggressive Titans would send a portion of their adult population to roam around, cause mayhem and collect valuables (especially artifacts), while Titans that are associated with good things would mostly keep to themselves.
To keep Megabeasts relevant even longer, I think Necromancers should seek out their corpses once the Megabeasts perish and reanimate them. What better pet would there be than an undead Dragon or Roc?
Historical Figures that are good at animal training might even try to tame Megabeasts in worldgen. If they tamed one, the rest of them would be easy and you could have situations where the Elves use Rocs as mounts.
Sorry if this is sort of rambling, and again, there's a good chance people already suggested it, but I wanted to get people's opinions on it. So please tell me what you think!