Botany - Because quite frankly the plants in DF are lackluster. While you can do a ton of interesting things with animals, what is a modder of plants to do? All trees are basically the same. Hell, even the ability of a signle, big tree to create multiple units of wood would be nice. But also, what about more fantastic things? Trees that can fight? Or scream out alarms? What about plants that could let you make more interesting substances, such as healing salves. Herbs would be fun as well, so food can get even more complicated and interesting.
Chemistry - Ties in with plants, obviously, pretty well. Also, lets you explore more stuff like poisons and the like. Yeah we have some, but none are made, they just kinda exist. A nightshade plant could make nightshade poison, but that's it. What if you could make more complicated ones? And Antitoxins? And Antivenoms? What if you mixed root of zibzab tree and powdered quarts and it made something that would burst into flame at exposure to sunlight? How could that not be fun? Or even a rare plant whose extract, when dumped into water, freezes the water? Or maybe even a special plant that, when turned into an alcohol, would let a dwarf be able to not sleep for 3 seasons, but then have to sleep immediately at the turning of the next 4th season, no matter where he happened to be.
Zoology - We'll have a lot more tags and things to be able to create things like skunks and armadillos and even moles. How could would it be to have like burrowing animals suddenly show up inside your fort? Or some subterranean snakes? How about making fantastic beasts a bit more interesting, such as werewolves that are non aggressive humans most of the time, then turn into killing machines for a week at a time? How about having Unicorns be more than just Horses iwth a greater combat value? Pegasai! That would be cool.
Living things, people, that's where you'll have a lot of interesting interactions.
But seriously, if nothing else, Botany. For all the sciences I think it's received the least attention.