One problem with 1/72 scale plastics is the lack of science fiction models. However, there are good miniatures of very modern armies, and these can be used for mid-tech science fiction forces. Also, there is a wide range of sci-fi minis available in 15 mm metal, and the aliens in many of them can work well with 1/72 scale. I'm currently trying to create science fiction forces with 1/72 plastics, and here are some examples of what I have so far.
Here is a 1/72 scale Caesar modern French soldier and a Ground Zero Games 15 mm alien, the Ixx. Due to the massive size of the Ixx (for 15 mm) they can be used quite nicely together with 1/72s. In this case, they are for my force of alien tribesmen (tribesaliens?) led by human officers who sort of have a French Foreign Legion vibe. The FAMAS rifle looks pretty futuristic too.
More 1/72 scale Caesar modern French with FAMAS rifles. These will form the backbone of a mid-tech sci-fi army.
This is a Ground Zero Games missile launcher drone along with another Caesar French soldier with an APILAS missile launcher. The UGVs (unmanned gound vehicles) will be a nice sci-fi touch to the heavy weapons teams.
Now here's something for them to blow up. These are Trumpeter 1/72 scale BTR-80 models. Modern wheeled APCs can be used in a variety of sci-fi settings. Wheeled APCs make a lot of sense, even on alien worlds, since they're cheap and low-maintenance, and sure beat walking when nothing higher tech is available! These will be used in an opposing force to the Caesar French.
These are Orion 1/72 modern Russian soldiers. When I first got them, I thought they looked horrible and were extremely poor quality. However, after a primer undercoat, they don't look so bad. A simple paint job and a wash can bring out some good detail. Again, these will be used as the enemy for the Caesar French. Those poor Russians, they never get a break!
I will probably use Fast and Dirty 4 for the ruleset for now. When Ambush Alley's sci-fi rules come out, I'll definitely get that.