The UFO TV series was recorded in 1969, but aired in '70-'71. This was the era of early space exploration, the Vietnam war, hippies, hair helmets, and a certain color palette (beige, brown, orange, olive, silver...). I was born in the late 70's, so I missed the spectacle.
Perhaps they were more optimistic about space travel back then, with the space-race advancing things so quickly. The series takes place in the future, 1980, and we have lots of spaceships and moon bases, all operated by a secret UFO-hunting organization known as SHADO.
The whole setup reminds me a bit about X-COM, and I've heard other people make that connection too. RebelStar II actually has an "Eagle" ship from Space 1999 (the series which followed UFO) in it. UFO's SHADO hunts down aliens with "Interceptors" and send ground troops to loot the crash sites, and there's a constant budget struggle.
The Aliens in UFO are a bit plain - it's just men in spacesuits. The series only lasted one season, so we never found out much about the aliens. We are told early that their humanoid bodies are made from harvested body parts, (including the brain). The spacesuits are filled with liquid, perhaps to facilitate their mode of space travel. It's possible that the aliens are a dying race of (machine?) minds, and that their invasion attempts are desperate attempts at survival. I could imagine that the aliens are busy on other fronts, assimilating other aliens out there.
Science-wise, the writing was pretty weak. There were also plot holes, and characters changing personality from episode to episode. On the plus side, the stories were hard to predict, perhaps due to the often dark endings and unintentional red herrings. Many of the episodes had a very typical plot theme though.
The vehicle/ship action scenes often doesn't show the surrounding environment. There are a lot of recycled shots of ships firing and exploding. Space 1999 did a lot better in this department.
Visually, the memorable characters of the show are: Straker, the boss, with his brown/biege high collar overall suit, and eggshell hair. Gay Ellis, a female operator at the moonbase, wearing a silver suit, and purple hair. Gay Ellis is probably everyone's favorite though, even though her character is quite undeveloped. The Interceptor ships with the one-shot nose missile are initially pretty fun, but the recycled stale shots of them are not.
With these designs, I wanted to play up the toy-qualities. The Anderson (Thunderbirds) stuff seem to be oriented towards playability like that. Unfortunately, a lot of these vehicles are too big for an X-COM game, which is much more about foot soldiers and character development. Also, a mecha-interceptor would be overpowered, and not fit with the scale. In X-COM, the tanks are small and cute.
UFO only features one type of alien, but it's conceivable that they have assimilate other alien races. I played around with X-COM analogies here. A tree-way conflict could be interesting, and it might explain why the aliens can't go all in - it would leave them open elsewhere. Maybe earth is just a strategical opportunity for them.
The earth uniforms seem to be beige and brown, and so are the fishnet-themed naval uniforms. In space, the suits are silver of course (with occasional gloss yellow). I'm not sure what to do about the collision of beiges. Maybe the land uniforms need to be brown or green to indicate top soil or vegetation. A cold gray... might be too close to the silver of space.
The whole moonbase setup is a bit strange. The Moon orbits the Earth, so why doesn't the aliens just attack from the opposite side? Traveling all the way from the Moon to the other side of the Earth must take a long time. SID, the radar satellite probably needs several sensors to see in all directions too. I like the space setting, but there also needs to be an organic, human side for the threat to feel real, so both the Moon and the Earth is necessary.
What if the entire story takes place on a lush colonized moon, somewhere else in space? This would solve a bunch of problems. The human population could be smaller, explaining why human kind doesn't bring all it has to bear against the aliens. There could be moon-like wastelands, lush forests, and cities with people. Several defense plans against the aliens could be competing, with the SHADO project just being one. If they do badly, funds will go to the other projects (which will fail and result in a bad end).
A terrible viral infection or somesuch could have threatened Earth, but a few million people managed to break away from earth and reach the colony using a fancy one-way method of transportation. This way, the reader/viewer/player can be left to wonder about the state of the Earth, and feel like, perhaps the colony is the last basket of eggs. If necessary, all of the colonists could be artificially grown humans (e.g. all female, or all fit, or whatever). It could be coupled with the virus and transportation plot somehow. In the UFO universe, it's seems possible to transport a mind over large distances, into a fresh new body.