Yeah that sounds interesting. I read like tons of explinations regarding, but this one seemed like the only one that provided "the bigger picture" of the Shivans, while pretty much all other campaigns were basically making a "spiced up" version of FS2 - like another invasion, just by bigger ships, omitting the possibilty that the Shivans had something completely different in mind. Stuff like that inspired me to write a (more campaign outline) fanfic with very, very big Shivan stuff, and it's one of the reasons I'm asking for these old things. 
I know it's been like forever ago, but is there something you remember besides that what PS story was like aside "The Shivan invade.", or even have some notes/design doc used back then?
Well the basic outline was in that thread for the acts, basically someone following up on Bosch/ETAK (Kepler's take is basically using ETAK to scramble Shivan communications/thinking) to basically create the "alliance" with the Shivans (i.e. how do star systems need to be arranged/maintained for Terran and Vasudan subspace traffic to happen). Overall though heavy subspace use for FTL is bad (accelerating the heat death of the universe type bad) and the Shivans basically wipe out species using it and have to re-arrange nodes and systems to keep things as natural as possible.
Some plot twists like finding out that the Vasudans were the Ancients, basically a colony of Bosch-likes who tried to make an alliance with the Shivans and set themselves to a pre-warp civilization. So co-existence is possible, provided you don't use subspace and remember to not use it.
Basically the Shivan hornet nest gets kicked with the communication attempt and attempt to find out what happened to Bosch under the guise of exploration fleets (which only one vanished) which results in the opening being "Shivans invade" through the series of Maginot Fortresses.
You then have the ticking clock of the Shivans doing stellar engineering in GTVA space and the last-ditch attempt to kick the Shivans in their teeth by trying to take out a central dyson grid structure (Shivans using stars for their communication hubs/mass redistribution for subspace). The "shivantown" Volition mentioned.
Basically you get the choice of: we can keep trying to kick the shivans and force them to blow up stars away from GTVA space so that the GTVA can keep using subspace. Or realizing that maybe subspace isn't worth the cost since who knows how many civilizations the Terrans and Vasudans are dooming by blowing up Shivantown and using ETAK to convince them to focus their subspace maintenance elsewhere.
So no "super evil bigger badguys" just FTL ****s up the universe, and finding out that we're basically an annoying itch.
Very much a case of the GTVA can win every battle, but not the war. While a destroyer might outclass a Lucifer, it can't stop a Sathanas fleet from coming in and re-arranging the furniture.
Gameplay wise though definitely a "bigger FS2" in that you have bigger ships (the small moon size stuff Volition mentioned they wanted to do), superstructures, etc. But the big thing is making sure that the player can interact with it. So if there's a Ceres sized Shivan ship, the mission might be taking out the comm relay with a destroyer group in a hit and run since outright destruction isn't too viable.
