You should still get all the sieges, but there are quite a few monsters/mega-beasts that trigger off higher populations. Some you will probably encounter anyway, mainly as they cross your particular area of the map.
Is it possible for those monsters to be changed at some point in the future to also trigger off something like exported wealth so that those who enjoy high fps can also have some fun!! with them?
Most will be triggered by exported wealth. The "world enders" come around the 1-million marker.
TLDR: gui launcher is screwing my save when I try to change graphics pack. Solution make sure you have preferred graphics pack selected when genning world, however may have missing content. For this reason I suggest phoebus or spacefox b/c loss of one glossable rock less significant than loss of creature(s) or scroll roller items for libraries.
As an extension to my previous post. The game I have been playing still has some issues. I am playing in the spacefox tileset
1. I was unable to blood helmets as plant seed and have not been able to request them through the caravan. It is possible regarding inability to request that my civ simply has no access. However they are listed as a legitimate crop in my farms I simply have no spawn.
2. I do have access to some of the stones I know are required for Mythos intractions ie brimstone are available to me but i think that those are in df vanilla too. I still have access to all buildings/reactions thus I would guess the issue is purely in the addition of the one stone type and in the item_raw folder.
I hope this was helpful I love what you are doing and anticipate many hours even in a slightly more limited Mythos
Brimstone is more common in Dark Ages, there are no other changes to mineral raws -but this should be working as intended.
Blood Helmets use "spores" instead of "spawn" that cannot be eaten or cooked, but they
should also be working correctly, if your civs have access to them.
To quick fix the big issue (for now), copy and paste the "item_tools" raw from a (pristine) Spacefox graphics folder into your saved game raws. You shouldn't need to gen a new world that way.