A lot of the things that used to make dwarf fortress dangerous are gone now. Will we ever see a return of the Boatmurdered era? I am asking this because I am interested in starting a succession game but I am afraid it will be rather boring without a substantial threat of death (which there really isn't right now).
I modified my mega beasts to be more dangerous. For example, I made them all much larger. In my mind, if a Giant has a size more than twice that of a Dwarf, then a Dragon should have a size more than twice that of a Giant. In my imagination, the Dragons are akin to Smaug from Tolkien mythology. The Titans are akin to the Titans from Greek mythology. The Hydras are akin to the Lernean Hydra in Greek mythology. Finally, the Bronze Colossus is akin to a construct, or golem, of at least the same size as a Titan. Imagine the Colossus of Rhodes getting up and walking around, smashing things.
That said, I gave Dragons fire immunity in addition to fire breath immunity. This oversight struck me as somewhat silly. I also made Dragons capable of speaking and learning, as they have always struck me as intelligent creatures.
I have also modified the Bronze Colossus to give it damage reduction equal to half its size. As a giant construct, or golem, it should be incredibly resistant, almost immune, to damage up to a certain point. At this point, it should break horribly. Thus this change plays nicely with the sever on break attribute and their incredibly regenerative abilities.
I have begun thinking of modifying the semi-mega beasts and the basic races to more closely align with my perception of the game. If you have any additional suggestions for mega beasts or other creatures, feel free to contribute.