I had a Civ universe idea last year: to introduce UFOs. The identity of the objects could remain hidden, so not necessarily alien.

Like an Area 51 sort of thing?

Could be. The fiction could be open to all sorts of interpretation, from a result of black project technology and military abduction, or a mass psychic projection, or a hidden ancient civilisation under the ocean, or alien probes.

This sounds like a neat idea. Since it's Post-WW2 esque, this is the perfect time for me to jump in with an expy of the Soviet Union. The plots you suggested are also pretty good and interesting.

Most of these ideas have been popular (to the extent that "popular" describes the Civverse) throughout the previous occasions where there has been activity in this area, and the few Civ fictions that currently exist are based around a 23rd-century-esque theme with rival megacorporations, so building the setting around this could be good start.

The geography of the planet should be open to change given that there are almost no details about it anyway. A major point in previous discussions about Deisna was that it should have multiple sapient species while being fairly realistic about it, hence the idea of myriad islands and small continents where creatures could evolve mostly independently. The other idea was mass genetic engineering for cosmetic purposes, which may still exist, but not everybody may want that for their creations.

We would probably need the Civ universe to contain only one species. Users would create their different nations' buildings and vehicles. It could be like the Civ Stage in Spore in that sense.

I like what's been suggested, I'm just wary that the idea might be abandoned again.

I'm personally not for a one-species setting. We had the idea that if a species was to be created in the Civ Universe, then it would be open for every user to use if they wanted.

This would be an amazing idea. Though we'd need finite amount of free space for new civilizations.

Maybe a huge planet (5xE-mass) populated by civilizations establishing first colonies in the system? Or two terra planets, both on the same stadium of development.

If the fantasyverse is anything to go by, space isn't going to be a problem even on an Earth-sized planet (and I don't think most people are likely to make creatures that are suited to living in 5g, or would it be an artificial planet with just a crust around an Earth-mass black hole?).

As for a double planet in which both bodies were inhabited, in that case it would be reasonable for even a 20th-century-equivalent civilisation to engage in frequent space travel between the two; it's just that in our world there's little practical motivation to go beyond low Earth orbit. So in principle that could work and the planets wouldn't be isolated, even though we do intended for there to be relatively little interplanetary travel here otherwise it'll be just another scifiverse.

See Wikipedia:Rocheworld, it maybe practical for even some kind of aircraft to travel one to the other. One might even find away for less advanced planetary civilisation to cross! The Roche limit is small because the mass of the two bodies is the same, though its unstable to our Civ planets unless it turns out they are ancient alien constructions or a very unlikely chance with just enough time for civilisation to spring.