Firstly I would like to say I have read that increasing megabeast size during world gen can increase the number that survive and that come to attack your fort but I would like to propose a possible remedy for them arriving with mortal wounds and dying on the edge of the map.
The remedy is based on the idea of removing all the damageable parts during world gen then giving them back when its complete. Thus keeping them out of harms way.
I have tried generating worlds with creatures that have their body tags removed but it crashes, so I tried with just having a [BODY] tag but again it crashes during gen. So now I have just [BODY:MOUTH] and it gens fine and lets me add those parts back with no problem. One test I did was to do this to Dwarves and remove the max age tag, in legends lots of the dwarves have bashed mouths and after adding all the parts back they run fine in adventure mode, so bodies don't seem to be necessary for normal activity as strange as it sounds.
Now my question is over what difference a bashed mouth makes, can there be deaths or is it just superficial, are there any superficial body parts? I only ask because I haven't ran a fortress long enough with these changes for megabeasts to turn up and probably wont do for a long while, busy with an existing fort. Is there a quick way to trigger megabeasts?
Just thought it was an interesting potential solution to an annoying problem.