The 3I's Long Range Plan
Let's start with the assumption that all information given in canon is accurate, that the leadership of all polities are rational actors who are motivated by national self-interest, and that no one is holding an Idiot Ball.
https://tvtropes.org/pmwiki/pmwiki.php/Main/IdiotBall
The 3rd Imperium is in deep doo-doo, and has been for a LOOONG time... destroyed and gobbled up by the Zhodani, Vargr, K'Kree, Hivers, Solomani, and Aslan.
Deep trouble, not really. The Imperium's only aggressive neighbors are the Aslan, Vargr, and Solomani. The Zhodani's wars are to check Imperial expansion toward the Consulate.
Really simple: Geography!
The 3I is in a vulnerable position, but only if the other polities act in a highly coordinated fashion, which they don't and have no reason to besides 'Let's get Impy." This doesn't even include the difficulty of coordinating battle plans over sectors between different species.
In the end it would shape up to be the Hierate and the SolCon with a Vargr sideshow against the 3I. The fronts would be the Spinward Marches, Daibei, Coreward raiding, and the Solomani Rim, with the aggressors banking on the IN dispersing its fleets so that their inferior fleets can defeat the IN in detail. This places their success or failure in the hands of the IN. This is a bad strategy, and the war planners of aggressors would know it.
The 3rd Imperium, or any alternate polity occupying the central region of Chartered Space, is surrounded on all sides by competing polities. Even before 1105 it had already reached the practical limit of expansion...
The 3I has not reached the practical limit of expansion, it has reached significant obstacles to that expansion. It could conquer the 8 sectors between it and the K'Kree, and pick up Reavers' Deep Spinward of Daibei. That's 9 new sectors added to the 3I's current 21 full/majority sectors, an increase of 42% without fighting a war against any of the other major polities. The 3I could conquer another 12 sectors of human client states and minor races to the Spinward of the Spinward Marches, so the 3I could more than double its territory without a war against a major polity, and have unlimited access for expansion to Spinward.
After digesting these 9 new sectors, the 3I could then turn its might against Mendan, Amdukan, and Arzul, and turn them into industrial centers and staging areas for the conquest of the wild frontier of Ktiin'gzat and Mugheent't. The 3I now has a wide open corridor 3 sectors wide to unlimited expansion to the Coreward and Trailing.
Contrast to that the fact that all the surrounding Major Species/Polities have virtually unlimited room for expansion away from the Humaniti sphere.
It is unavoidable that these other polities will expand and expand, while the central Humaniti region remains trapped...
No, the 3I just needs to overcome some challenging obstacles to create an access corridor to Coreward and Trailing, and it already has access to Spinward. The only polities making a concerted effort to expand are the Solomani and the Aslan.
As the other species grow stronger and stronger, but Humaniti reaches a state of diminishing returns, it is inevitable that the Humaniti sphere will be overwhelmed and divided up between its neighbors and cease to exist in any practical sense.
It is absolutely not inevitable. Even if the 3I did not expand, it would continue to increase its technological edge and industrial base. Its rivals would have to triple in size to threaten it, and it would still have the technological edge.
Anybody got any ideas how to deal with this issue in game, and perhaps allow the Imperial Authorities a little shut eye?
THE LONG LONG LONG LONG LONG RANGE PLAN
MISSION:
Secure the availability for unlimited expansion and ensure the 3I maintains unmatched quantitative and qualitative military, technological, industrial, and economic capability over all other major polities combined.
OBJECTIVES:
Conquer, colonize, and annex Ktiin'gzat and Mugheent't sectors to secure unlimited access for expansion to Coreward and Trailing.
Conquer, colonize, and annex the Trojan Reach - Mavzog Corridor to secure unlimited access for expansion to Spinward.
THE PLAN:
Phase I, Preparation:
Initiate an R&D and industrial development plan to make TL 16 the standard throughout the Imperial armed forces and then throughout the Imperium, and continue R&D to achieve TL 17.
Initiate the long term increase of industrial capacity to support military requirements for the future conquest and colonization of 25 target sectors.
Initiate a population increase program to enlarge the military recruiting pool and provide populations for mass colonization.
Subvert Vargr polities in the Meshan sector so that they may be used against Mendan sector and hostile Vargr factions.
Subvert Vargr factions along the Coreward border to turn them against other Vargr.
Reinforce the Domain of Deneb as a fortress domain and develop it as a forward industrial base and staging area for the assimilation and annexation of the 12 sectors from Trojan Reach Spinward to Chiep Zhez and Mavuzog.
Reinforce Antares Sector as a fortress sector and develop it as a forward industrial base and staging area for the conquest and annexation of Mendan, Amdukan, and Arzul.
Reinforce Daibei Sector as a defensive bulwark against eventual Aslan aggression.
Develop a wide ranging intelligence network throughout target sectors.
Infiltrate and subvert populations and governments within target sectors.
Initiate subtle colonization through immigration over time in target sectors.
Phase I, Diplomatic Initiatives (concurrent with Preparation).
Acknowledge current Zhodani-Imperial border.
Acknowledge current Solomani-Imperial border. Subtly make the Solomani aware of increasing Imperial frustration with Aslan aggression. Play on Solomani anti-alien sentiment to drive a wedge between the Aslan and the Solomani. Play on shared Solomani/Imperial heritage to decrease Solomani hostility between the Imperium and the Solomani Confederation.
Increase trade links with the Solomani Confederation.
Build positive relationships with the most stable of the Vargr polities along the Coreward border.
Phase II, Construction and Staging
Build invasion fleets under a variety of plausible excuses such as anti-pirate duty and the protection of Imperial citizens living outside the borders of the Imperium.
Assemble assault armies, occupation forces, and planetary development elements to prepare conquered worlds for follow-on colonial populations.
Propagandize the Imperial population to motivate them to support military action against the aggressive violent Aslan to Spinward and the treacherous raiding Vargr to Coreward. Generate media programming featuring bold dynamic Imperials conquering strange new profitable worlds, defeating villainous non-human sophonts, and improving Charted Space by bringing it under the benign rule of the Imperium.
Set up world development and exploitation contracts with the appropriate commercial entities.
Phase III, Invasion:
Create a pretext for war against the Vargr Coreward of Antares.
Create a pretext for war against the Aslan in the Trojan Reach.
Invade target sectors under a variety of excuses, like anti-piracy actions, border conflicts, assistance requests from client factions, Aslan attacks on innocent worlds, protection of commercial shipping, restoration of order, etc.
Phase IV, Ongoing Conquest, Occupation, and Annexation:
Import millions of Imperial citizens to conquered worlds using a variety of incentives. Create new noble titles and fiefs, and let it be known that they will be granted to those who distinguish themselves in the service of the Emperor during this great tide of history. Award impressive land grants and tax breaks to soldiers who will settle conquered worlds. Offer criminals freedom and land if they will voluntarily settle conquered worlds.
Develop industrial centers in the conquered sectors to support the armed forces and the new colonial populations.
Buy off hostile factions like independent worlds and pocket empires with lucrative contracts to supply the ongoing colonization efforts.
Colonize, develop, and annex the new conquered sectors.
CONTINGENCIES:
War with the Aslan Hierate:
Make a permanent peace with the Solomani Confederation by ceding the Solomani Rim to the Confederation and relocating all non-Solomani to the new colonies to the Spinward and Coreward. Betray the Vegans for the Greater Good (tm). Make common cause with the Solomani against the 'alien' Aslan. Conquer Reavers' Deep, Ealiyasiyw, and Hlakhoi from the fortress sector of Daibei, securing the Rimward flank of the Spinward conquests. Devastate Kyusu and the Dark Nebula sector, then cede it to the Solomani. Allow the Solomani a free hand in Ustral Quadrant sector and encourage them to expend their strength tearing off sectors from the Hierate. Make conciliatory noises to the Aslan, pointing at the Solomani as the primary aggressors against the Hierate. Play the Solomani and the Aslan against each other so that they will not form a united front against the 3I for centuries to come.
War with the Zhodani Consulate.
Conduct a holding action and make diplomatic noises about maintaining the current borders. Strike at Zhodani military bases and industrial targets to weaken their ability to project force, but always be willing to restore the status quo with an armistice. Steadfastly maintain non-aggression with respect to the Sword Worlds, and offer them their worlds back if they ally with the Imperium.