I would like to see the Hauling Arc come after the Caravan Arc. Goddess knows that far too much space and time is dedicated to hauling...but hey, if future Drives are dedicated to other things, like Content, here is what I want to see:
MEGABEASTS
Phoenix
a flaming cockatoo. Most likely avian candidate to make the Roc into a Semi-Megabeast, considering that a Phoenix is on fire, has the ability to be reborn, and is likely to be just as large. Probably fairly easy to implement, since it is just resetting this particular beast back into egg form or perhaps having it become a chick again in a blaze of glory. Either way, this bird is probably the oldest Megabeast in existence, if it were born in the first place.
Kraken
Oversized cuttlefish, which tend to be rather curious about things they find, including wrenches, bottles, and dwarven steamships. Only difference, is that a Kraken is large enough to take things from the surface, like ships and unlucky beach-goers for later investigation. It would be interesting if Toady could make tentacled creatures to be able to limbs extending from the main body for several tiles, with it being possible to sever them. Would make for interesting combat possibilities for this megabeast, and allow some of us to run a sushi bar, much like Order of the Stick's hydra cuisine.
Sphinx
A guardian lion that has gone Egyptian. Why not? At least you can then be a monument. More importantly, the Sphinx is known for speaking riddles and rewarding people who succeed, and making a snack of the people who don't. Tends to appear in front of a Dwarf Fortress and tempt the population with promises of pig-tail socks. Never before has an oversized cat been so content. Perhaps the Toady One could create a riddle generator in conjunction with the Sphinx, in part so that more artsy dwarves and pansy elves could try to seem witty, when not confronting a certain feline.
SEMI-MEGABEASTS
Cockatrices, Basilisks, Catoblepas, and Gorgons
All four have the ability to turn other living beings into stone. Cockatrices do so by making contact with someone - but wearing gloves and clothing across the entire body should prevent stoning. Catoblepas are essentially large bulls, and they have some sort of breath that causes people to solidify. Gorgons are probably the most dangerous, considering that someone has to only look at a Gorgon to be changed into stone. However, being blind, closing your eyes, or otherwise not looking at a Gorgon would prevent this effect. Gorgons are the smartest of these creatures, considering that they are women with snakes for hair. Basilisks can touch, breath, and stare people to stone, plus they are venomous. However, Basilisks are as bright as a bag of rocks. Since they all are about the same ability, only with different means of delivering it, they would make sense for a Megabeast/Semi-Megabeast drive for Toady to focus on if this entry wins.
Vampires, Werebeasts, and other diseased social rejects
The legends surrounding the vampires and werebeasts often involved some sort of enchanted disease that defies thermodynamics and evolution, clearly putting the notion of science to rest. Not so much in Dwarf Fortress, where being a Werewolf is apparently not a disease, but something you are born as. Correcting this would probably take some work, but might be helpful for coding in other wonderful ailments. I bet that the change over to a Werewolf would alter the personality of the victim, as would vamprism. So expansion of disease spread, effects, and generally improving the implementation of the monster population would be the benefit of this path.