The problem with making a generic space mod is that it's not something people will recognize. You can replace elves and goblins with googoochus and slarphnaxi but then the player is constantly asking themself which is which. From a literary perspective, it's probably better to have something like space goblins and space elves, or make aliens based on earthling concepts, like rabbitoids, squidoids, rockoids or vikingoids. But for DF to feel spacey, you need to do that for an entire planet's ecosystem, so you have the same problem coming up with crops and animals. But to be fair, DF has its share of alien crops and animals, things like plump helmets, sun berries, crundles and manera.
One of the more interesting things you could do is try to build planets based on alien geology. There are a lot of real life planets and theories to go on.
Titanoid - Saturn's moon Titan is famous for being the only other body in the solar system with liquid on its surface, however the lakes and rivers of Titan are made of hydrocarbons instead of water.
Subsurface Ocean - The moons Europa and Enceladus are covered in ice, but may have liquid water between the icy crust and the rock below, heated by friction between the layers and volcanic vents.
Ice Ball - A planet that is predominantly solid water throughout, with no rock/metal even in its core. Several moons and Kuiper belt objects in the solar system represent such a construction. In the solar system, ice balls are so far from the sun that water is naturally in a solid state, and liquid water or steam would be as rare as magma is on Earth. Different types of ices may exist, for example, Pluto has water-ice mountains which may have been carved by methane-ice glaciers. Some of these bodies have cryovolcanoes, which spew liquid water instead of magma. Some cryovolcanoes may be caused by chemical reactions instead of heat.
Cannon Ball/Chthonian - Planets made predominantly of iron, as opposed to Earth which is mostly silicate. Mercury supposedly has a very high iron content, but the surface is still rocky. Some of these planets might also be Chthonian planets, gas giants that have been stripped of their atmosphere, or even rocky planets that have lost their crust due to impacts or unknown causes.
Carbon Planet - Carbon planets are defined as planets where there are more carbon molecules than oxygen molecules. No such bodies have been conclusively identified, but they are predicted to form around carbon stars. There are carbonaceous asteroids in the solar system, which might be similar. Chemistry would be completely alien on such planets.
Torch Planet - Many planets exist very close to their stars. This can get hot both because of the star's radiation and because of tidal heating from the star's gravity. Some gas giants have observed with clouds of silicate, meaning it's so hot that rock has not just melted, it has evaporated. One might imagine an equilibrium temperature where rock exists as solid, liquid and gas, as water does here on earth. Unlike water, "magma" can be one of several types of rock, each with its own slightly different melting points and chemistry.
It's worth pointing out that Earth's atmosphere and some of its geology are influenced by the biosphere. Earth probably used to have a CO2 atmosphere like Venus and Mars and an abundance of anaerobic microbes, but the appearance of CO2-in-O2-out breathers removed CO2 from the atmosphere and created air that was poisonous to most other lifeforms at the time. For a while, this drained all the carbon out of the atmosphere and created a massive ice age, until O2 breathers developed and balanced out the atmosphere into one more or less like the atmosphere today. So some of the chemistry we see on lifeless planets and the chemistry we take for granted on Earth could be completely different on planets with alien biospheres.