The real question is how ambitious you'd like to be.
For a small mod, you could, for example, add various syndromes to each type of booze, making things a touch more interesting and meaning that it really did matter what kinds you made. That's pretty simple and would really only take a few minutes.
For a slightly larger one, you could create a new race that can invade the fort, starting with an existing creature and either changing it, or just working with it as is. Create an appropriate entity file and go from there. There's nothing stopping you from making kea-men into a thieving civ, for example.
Alternatively, you could take a different route and create a new industry. For that you'd probably want to come up with a custom workshop or two and a list of reactions to use there. After that, you could do almost anything. Perhaps your dwarves love cheese and make different kinds out of different rocks. You could end up with some preferring blue microcline cheese, while others prefer green olivine cheese. Quite easy to do, but requires you to play around with a few more things, like fake animals.
Really though, if you can't think of anything to do, a good starting point is to just look carefully at other mods. After a while, you start to get the feeling that 'this is really nice, but I kind of wish that it was just a bit different'. Hold onto that thought, because it's the start of some kind of modding idea. I was poking through the MLP mods and liked the gem lens ideas. I didn't want to use their magic though, and I ended up creating a new creature that can be 'coloured' in order to transform into different castes, each doing different things. Superficially similar starting point, radically different result.
Have fun.