So I had fun watching empires rise and fall, and even assumed control of a few empires (using Emperor mode). I was about to quit until I discovered something...
You can play as the Neutrals.
I should have suspected they were playable, since all empires were descended from the Neutrals...and because there was a big button that appears if you click on a Neutral planet.
Neutrals don't have flagships, or indeed any kind of ship whatsoever. They can't conquer any planets and have to rely on empires "transcending" to expand and gain points. However, the Neutrals cannot die. So that's a point in their favor. Even if a faction conquers the whole galaxy, the Neutrals still lurks in the shadows, ready to take over planets when the galaxy-spanning faction decides to transcend.
The Neutrals still have access to spies, and I use them to try and manipulate the other factions. Negotiating for Peace with the other Empires don't seem to do anything (it's possible the other empires aren't even able to recognize peace with the Neutrals). Causing Riots can be pretty effective, but it just create new empires...it doesn't cause planets to revert back to Neutral control. So the only action I can sensibly take is "Stealing Tech". My hope is that if I steal enough tech, it should become very difficult to conquer Neutral planets.
Right now, the Neutrals have a Tech Level of 3 (they started off as 1). For comparison's sake, it takes a Tech Level of 9 to transcend. Since new empires get the same (or lower) tech level from their ancestors, I'm trying to see if I can get up to Tech Level 9, so that I can induce Neutral-descended empires to transcend immediately upon existence, thereby causing their planets to revert back to Neutral-owned.
...playing as the Neutrals would make for a very odd Let's Play.
EDIT: The Neutrals went up to Tech 8, before the largest Faction ( also at Tech 8 ) recently woke up from a Blood Feud into an "All Will Be Ashes" stage. It then immediately self-destructed, with some weird message to the effect of "{FACTION} has cleansed the galaxy of all blood." I, obviously, inherited all those planets. The second-largest faction at that time was at Tech Level 7, so you can expect the galaxy expansion to slow to a crawl. (Some empires did spawn from the Neutrals with a Tech Level of 8...but then again, they have to face off against other Tech 8 Planets).
You still have to worry about government type though. The Neutral Empire has entered into a period of degeneration, and as a result, destroyed an artifact and went down to Tech Level 7.
EDIT 2: Recovered myself back to Tech Level 8 and started seeing some planets turn Neutral after a "red" explosion. Is there another way for Neutrals to gain territory other than through transcending then? (Well, I know that other random events [like supernovas and marauders] can flip planets Neutral. I also remembered seeing a message about the Eater of Life coming over to the galaxy...could that be related to planets flipping Neutral?)
EDIT 3: Game crashed on me, so I am unable to continue further. Stealing Tech doesn't seem to raise my level any further past Level 8. Either it takes too long or you have to do some "research" of your own before you can get to Level 9. However, I did see many empires (big and small) transcend. One empire transcended after 3 millennia and conquering less than 10 planets! A huge chunk of the galaxy (say, perhaps 1/4) is Neutral-owned, and expansion for the other factions can be very slow. This is probably the best I can achieve with the Neutrals. I declare victory and promptly uninstalled the game.
I also concluded that it was the Eater of Life that was causing planets to flip over to my side, after a specific coordinated series of explosions that was followed by the Eater of Life's taunts. It does seem that, in any LP that deals with the Neutrals, that you have to treat them as a force of Entropy (just as much as the "All Will Be Ashes" and the "Blood Feud" factions). Their goal is to destroy everything in its path, to leave nothingness and emptiness in its wake. However, the Neutrals are always doomed to failure because their sheer existence serves to incubate future factions. Under this interpretation, 'transcending' is a euphemism for 'mass suicide'.