Okay! So after working on them some more, I've got my first set of sci-fi improvement models ready!
Futuristic/Alien Arcology, Subspace Beacon/Alien Subspace Beacon, Psi Gateway, Wind Traps, Solar Farm
The "arcology" model was originally done for Civ 4 by dacubz
The Subspace Beacon, Psi Gateway, Solar Collectors, and Wind Traps were originally done for Civ 4 by Lord Tirian
I was playing around with some of the textures, which is why there's alternative versions of some of the models -- the Alien Skyscraper model from Civ 4 has nice and big flat surfaces, so it was fun trying out some of the other textures with it. The black 'alien' texture was the original one, but I liked how the one I used for the "Futuristic Arcology" gave it a kind of Star Trek starbase look. With the Subspace Beacon, I didn't really like the original yellow/green look so recolored it to appear more silver/blue, but I've left the files for the original subspace beacon in for those who might like it. I also tried the "alien" texture, and found it gave it an amusing Necron-like look, so included that one as well.
There's also an issue with the Wind Traps -- for some reason, the internal plate isn't appearing in the final model, although it shows up in Blender and Granny, so you can end up seeing through the model. I've been trying to fix it, but haven't made any progress, so I figured I'd just include it as-is for now until I can find a solution to the issue.
On the other hand, I'm pretty pleased with the Psi Gateway. I attached the gemstone sparkle effect to it, so while the gate is "active" it sparkles. On the other hand, when pillaged the energy field in the gate shuts down as well.
I've still got some more ideas for sci-fi improvements -- communications relays, research facilities, and industrial sites -- but it's going to take some more work to see if I can turn them into reality. Hopefully these ones will be useful for now!
EDIT: Art and improvement files are available
here.