I've re-read the lore in the instruction booklet for Sins of a Solar Empire and now want to sum up some existing ideas and the ones I mentioned before.
The Advent:
The rebellious Advent faction could be "the Silent". They are the ones who are not connected to the unity "naturally". Individuals who don't want to be part of the unity and use dark and obscure rituals, consuming allies to gain health and getting their abilities, draining life etc.
They want to be independent but they also want to find a new way of individual existence beyond the Unity.
The rare Silent could be the leaders of this new faction getting more members from non-loyal Advent who don't want to be part of the unity anymore. The Unity is a lie for them.
The Vasari:
The Vasari could split into a faction, that wants to support the old values of the Dark Fleet, means big ships and a new empire to face off the unknown menace. In later research, they can gain contact to the remnants of the Dark Fleet and could get very powerful new technologies. The second faction could be more the nomadic, hideous and the agile way. They want to search for another way to confront the menace, like ancient artifacts and weapons and not just facing them with big ships.[...].
The Dark Fleet faction is not interested in the warning beacons anymore. They feel scared but powerful enough to strike any incoming enemy.
The nomadic faction members want to hide themselves until they reach a higher form of technology and consciousness. They use prototype technologies which are not really tested in battle including tactics of other factions which they observed the past years. The knowledge of ultimate destruction during the past thousand years is always present in their minds. They want to find an answer, but not now. They want to find solutions for all kinds of threatenings, not just in battle.
But that's not the way of the Vasari anymore, some old veterans finally want to crush all enemies around them including this new traitorous faction with the power of a new Dark Fleet even more powerful than the old one. They need ressources, planets and heavy weapons to engage the enemy. They want to be sovereign, they want their last battle.
The Trade Emergency Coalition:
The TEC could split into these two factions: A trader faction, who just want to consolidate their trade order worlds. They are the ones, who have the old veterans. And the other one maybe a more paramilitary or secret service elite arm of the TEC, who want to strike back all attacking invaders with newest technologies but with less experience in direct battles.
This new paramilitary faction could have high precision weapons and tactics to engage even unknown enemies. Their goals are unknown, but they are a powerful force to reckon with. They don't stand a chance in a direct battle against the TEC fleet, so they use guerilla tactics and malicious ways to gain power and technology.
The Vasari nemesis faction:
I want to sum up some past ideas about the Vasari menace:
The Vasari's Nemesis race- It's mentioned in the back-story the Vasari are running from something, but what exactly is never revealed. Seeing this new race in all their horror would make for a nice continuation of the story and give the races good reason to rebel against themselves and ally with former enemies.
Nemesis Race: I like this idea. It could be a very interesting idea; however, I'd not put them in as a playable race (they'd either be horribly OP or uselessly UP). Rather, I'd script them as "super-pirates"; they attack non-Vasari because they're simply in the way, and Vasari because, well, they're the Vasari's nemesis.
But, what about something ENTIRELY different?
what about something not of normal space and time? something that could cause fleet-wide madness, and would be impossible to concieve, or something so horrible a mortal mind simply couldnt handle the ramifactions of it existing?
what about something in the vein of Lovecraftian Entities?
think about it. the Vasari, experimenting with Phase Technology in the middle of the homeworlds accidently breach into an area of the Universe where Non-Euclidean geometry is Rampant, and where the normal laws of Physics are merely a childs times tables in comparison to the true laws of the universe, and something from Outside Crawls in...
It relentlessly pursues all survivors, and destroys there beacons along the way, slowly awakening its brothers and tearing phase space open, making the stars right again....
EDIT; im talking a race of Astral Outside Alien entities. Old One style.
have them be dark, heavily alien composed of odd behaving strange matter, and heavily suffused with dark eldritch and arcane Technology and Dark science.
New Faction, different, never done before in a strategy (that i can think of..) and beleivably powerful.
plus playable if balanced correctly!
I had some ideas about this in the past:
[...]I thought of abilities like changing the user interface, that the player can't manage to do something and sees crazy illusions on the screen for a few seconds or changing the whole background to another dimension-like background to create an atmosphere, that makes you feel delivered on the enemy.[...]
I have something like an Infinite Hierarchy faction in mind. It could be a gigantic empire with a mysterious emperor on the top. Only a few individuals know this emperor, some are not sure if he really exists or if he died in the past, but all members fight for the Infinite Hierarchy. They are organized in many many universal spanning castes with their own leaders. Just a thought, the ideas above are maybe better.
General ideas:
An option for planetary governors, who defend your colonies if they're attacked. If you have too much planets, this could be useful. Maybe the governors should get a little reward for defending...
The defending forces shouldn't be very powerful, just powerful enough to hold the position until allied forces arrive. Otherwise there are the other defending points like hangars, space stations etc. so there is also the possibility of mercenaries. Maybe a system could be integrated to choose randomly between different tactics or ship-classes.
A temporarily government on a conquered planet, so that you have contact to your lost planet for some time and to weaken the hostile empire within. There should also be a counter attack to reveal the hostile temporarily government on a planet.
So this could be similar to a secret conspiracy on a lost planet. For a reward the conspiracy let you see the enemy structures or you can choose between different options of rebellious actions. This should only work if the cultural influence of the enemy is low and for important worlds the player had lost.
It wouldn't be like the existing cultural influence.
Scorched earth tactics: Once you know that you can't hold your planet, you can destroy all structures on the planet so that only a scanty rock is left. The enemy couldn't get the ressources/not so much ressources out of your former colony planet. There should be the restriction, that this tactic decreases the friendlyness of other empires very much.
This is not the same as leaving the planet and destroying the infrastructure, but also destroying the natural ressources of a planet. You could choose between virus bombs, so that it is not easy to settle life on the planet or maybe geological instability, so that it is not so easy to build up some good infrastructure. Something similar to this.
The astronomer Carl Sagan mentioned an Encyclopedia Galactica in his works. Maybe the developers could add something like this for background information about the factions, the ships, history and especially what happened the past years in the game. Something similar to the Mass Effect 2 codex. It would avoid the need of a campaign.
The pirates should get more different types of bases, like space hulks, old damaged and modified warships which can attack the player without other weapon batteries.
To stars in general, I would say that there could be star giants which are placed a little bit under the map and the battlezone and if a ship gets destroyed, it will crush into the sun below.
