I said when it started that the Stellaris [official site] developer diary would be interesting, and here we go. The latest update - the third overall - is about galaxy generation, what options it presents to players, and what they expect modders will do with the system post-release.
Your generated galaxies can "range from 200 - 1000" stars and can be made in a "spiral, elliptical and ring" formation. I quite like the idea of playing with a ring galaxy and trying to form defined frontlines with neighbouring AIs. Once you've made your decisions, the game handles the rest, generating a galaxy of randomly positioned stars, planets, space debris and other interstellar phenomena:
One of these are nebulas. Nebulas are visible on the galaxy map and often contain a bunch of interesting star systems with special rare resources. In a nebula you can expect to encounter some special events and experience certain penalties and bonuses that may impact your decisions when it comes to colonization and fleet movement.
It sounds as if every part of this will be exposed to modders to meddle with after the game is out, including the available galaxy shapes and which "initializers" - starting settings for players - are activated depending on the galaxy you create and the race you choose to play.
This is the kind of thing I was expecting to hear about after reading Adam's Stellaris preview at Gamescom, but I'm susprised by how pretty the two screenshots in the above linked dev diary are. I don't normally associate Paradox 4X games with lovely soft lighting.