Hm... even after removing megabeasts en masse (leaving the 5 semis and only 2 megas), I still get the same error, so sheer number isn't the issue. And I don't think that .17 bug is it, because it worked on a small world, but nothing bigger than 33x33. Even 65x65 crashes. So that's about opposite of the problem perhaps? I'm going to check if my upping the number of caves and the like will do something.
EDIT: And just encountered a new crash where it will do so during the history gen. Haven't touched the cave numbers yet, mind you, just added all the beasts back in. This was on an even larger map than before.
EDIT2: Perhaps just found a possibility. Going into the arena, I discovered two 'nothing' creatures... never had a game crash from that, but now I have to find out what it was that's doing it.
EDIT3: The 'nothing' spawns as a feralbosam. Arena still has normal feralbosams and the normal harbinger variety as options, and I don't see anything wrong in the raws.
EDIT4: Lots of edits, but finally found the error with those raws. Defined castes weirdly and duplicately and so on... seems like it was my problem. How a caste error crashes world gen, I do not know, but the reason the small region even worked was likely because the game didn't spawn any feralbosams, due to size.
EDIT5: Reeeeeeally odd error caused by numerous duplicate megabeasts causes naked mole rats, giant rats, cave crocodiles, etc. to replace entity civ creatures in adventure mode.... never seen that one before, but fixing redundancy fixed the problem.