We already know who's going to be in the base game. From the Community Q&A:

Geez, I'm sorry I clearly made a mistake. The factions above were only meant as possibilities during the story-driven campaign. I understand if you can play as practically any race in a custom game.

I was also wondering if anyone thinks the GalCiv guys should allow people on the forums to contribute in some way to the story. I mean, there's a lot of unknown's in the story, like where did the Drath go, how the Krynn Syndicate formed, everything in the 16 years since the humans fled.

I know games like Star Citizen do this but I'm pretty sure GalCiv doesn't allow fans to fill in the blanks. Unless that's for the "coming soon" stories page in the databanks.

For example, I'd do something like an Arcean trying to fit in the Drengin dominated galaxy, by maybe working for those feeble-looking Iridium as some muscle.

Anyway, what do people think happened to the Drath, or what Arcean survivors are up to, or any possibilities of stories in the 16 years?

No.

The Drath homeworld got exterminated during the Drengin/Korath invasion (first mission of the DA campaign). Frogboy hinted that the Drath might not be completely wiped out, but I doubt we'll see them as a playable race again.

Their databanks entry provides a little bit of information about that, but we'll probably hear more eventually.

Do we really need to know more than what the databanks and the timeline say? I would be the first to ask for more lore, but even I think, that we don't need to know every single detail. Some things are best left to the imagination. Plus, we probably hear more about what happened during the campaign.

Most likely rebuilding their civilisation and, more importantly, their army.

Agreed, I also expect the Arceans to show a tougher, more hostile side of their culture than in GalCiv2.  They might even be hostile, or at least very guarded, towards terrains.  Terrains?  Perhaps two factions?  Lots to see....

Why dont we have more information on this. ;(

There should totally be more civs! Moving from 12 to eight isn't fair. They will totally add more, but it will probably be DLC. (Which makes more sense, considering how many hints they've made about DLC and the Premium Founders.)

Did anyone like a minor race enough that they wanted it to be playable? I know the Snafu (evil squirrels) are a joke, but I like jokes.

I like the Akillians they have great abilities in the example mod. I like the Paulos.