Biography

The Tal'Rashii are are microscopic organisms that naturally form together into colonies for protection and reproduction, but must do so within a suitable host.  For millennia, they remained relatively unchanged, until the day the dust-infused Artlothans landed on the Tal'Rashii homeworld.  Upon merging with the Arlothans, Tal'Rashii colonies gained sentience within their new host bodies.  Being a deeply religious culture, the Arlothans interpreted this secondary consciousness within themselves as an Awakening of Spirit, and strove to meld themselves to the Tal'Rashii (The Arlothan word for "Benefactors").  For it soon became obvious that being bonded with a TalRashii colony not only brought the organisms sentience, it increased the strength, intellect, charisma, and wisdom of the host and colony.  With the Tal'Rashii and Arlothans working hand-in-hand, every Arlothan was bonded to a Tal'Rashii in just a few decades.  The Arlothan culture reached unimaginable heights -- the arts flourished, science advanced at an incredible pace, and peace became a reality.  Unfortunately, the Golden Age did not last.  Before the Arlothan-Tal'Rashii could truly become intergalactic species, a supernova wiped out their home system, leaving only a few scattered exploratory colonies that were not destroyed.  Of these, only one was sufficiently established to not regress into a dark age.  But the Arlothan population was too small to feasibly bring their species back.  The Arlothans and Tal'Rashii made a decision -- it was time for the Tal'Rashii to find a new host species.

Political Ideology 

Pacifists.

Gameplay

This faction will be played as the symbiote, not the host.  As such, whatever traits they possess stem from whatever hosts they might have found.  They have the ability to pull faction traits and affinities from those they have come in contact with, depending on the type and strength of relationship between them.  Every play through with the Tal'Rashii has the potential to be completely unique, depending on the mixture of minor and major factions they encounter and the dealings between them.  And since the benefits of having other factions maintain friendly attitudes towards the Tal'Rashii are so important, open conflict becomes a last resort by practicality alone.  

Faction Affinity

Symbiotic Relationship - Upon meeting a new faction, the Tal'Rashii gain one random positive non-militaristic trait possessed by the new faction.  As relations improve, additional attributes will be copied depending on the attitude level between the Tal'Rashii and the faction in question.  Should a faction reach a certain level of camaraderie with the Tal'Rashii, the Tal'Rashii have the ability to enter a dust-induced merger with the host species, allowing them to permanently acquire that faction's affinity.  Minor factions will provide one random trait upon first meetings, and an additional trait if the faction is absorbed into the Tal'Rashii civilization.  However, as relations decay, the anger of hosts associated with the now hostile factions heavily impacts the symbiotic relationship.  Positive traits are removed, and eventually replaced with random negative faction traits.

Population Bonus

Symbiotic Unity:  Increased influence for each non-Tal'Rashii faction represented in a system (populations per system are determined by level of symbiosis with each faction -- the higher the symbiotic relationship, the higher the percentage of that species within any given system in the Tal'Rashii civilization).

Faction Traits

None, until the Tal'Rashii begin interacting with other factions.

I'm not up to speed on what the new system will be for how the factions will perceive each other.  I know in Endless Space 1, it was called "Attitude" and it had seven levels to it (ranging from "Hate" to "Very Close") depending on how your relations were.  That's what I was referring to by camaraderie, just a placeholder for whatever the devs choose to use for the system in this game.  

And you're absolutely right on the difficulties behind the coding of it.  I have no idea if it would be possible or not for the devs to pull it off, but it seemed like a unique way to have a faction play, so I figured I'd throw it in the hat and see what came of it.