I love to play in long history worlds, many things that happened, many legends told and untold, many stuff to see in Legends Mode (well, especially if the exports will eventually be back in Premium), so many historic events engraveable by your dwarves...
But letting a world gen running for long eventually end up to have many downsides : so many necromancer towers that may eventually overrun the world (and living's civilization are not doing enough to stop them), so many human monarchs are vampires (and their subjects don't care enough about this), and last but not least : so many megabeasts eventually get killed by historical figures, or worse, very random war animals during their attacks.
Having some historical figures killing some of the megabeasts is great for creating epic stories to browse in Legends mode, but it's less fun once you are in game if there is almost no megabeast left to attack your fortresses, or to heroically strike down in Adventure mode. Dragons seem to be particularly vulnerable to this, they seem so fragile compared to other megabeasts, almost all of them are dead if I don't use minimally favorable parameters in world gen.
One solution available right now is to stop world gen automatically once a given number of remaining megabeasts is reached. But even with the max number of MB settings available in the basic world gen, it often end the generation before year 270 on Small worlds and smaller (and adding more MB in advanced world gen may unbalance the developpement of civilizations I guess). And even if I go to 400, most dragons are dead anyways.
Another idea could be to add a parameter where once you reach a given number of MB, their interaction with humanoid (whether it's them attacking civilizations, or humanoids going to their lairs to kill them) are strongly reduced or even stopped. So no more MB die until the end of worldgen.
It could look like something like this in the world_gen.txt
[BEAST_END_YEAR:100:80]
[BEAST_PLOT_ARMOR:90:50] (the first number being the plot armor strength, the percentage of interactions with humanoids that are prevented -100 means no interactions at all from now on to the end of the world gen-, the second number is the percentage of dead beasts from which the plot armor activates, like with BEAST_END_YEAR)
Do you have other ideas to allow generating pretty long world histories without worrying about having most of the beasties dead? (especially dragons, taking 1000 years to reach their full size is too long. Also, I wonder why they don't have wings, while cave dragons who are supposed to be their descendants who adapted to the life in caverns still have vestigial wings)