My idea for an AU, fused from a couple of tribbles by Drakensis over on the BT forums.
During the Davion Civil War, the Outer March decides to break away from the Federated Suns, with the Prince on Filtvet feeling that his worlds are no longer receiving any benefits from House Davion, while all factions are demanding resources from him.
Without enough heavy industry of their own, House Esteban establishes trading ties with the Taurians, the United Hindu Collective, and the Outworlds Alliance. After winning, Alexander Davion negotiates a peaceful secession of the Outer March, as it was an overall drain on the FedSuns, and he is exhausted by having to defeat the other Marches. The region becomes a refuge for many unhappy with the stronger grip House Davion has over the Suns.
Taurian industries, with a much larger market to work with, enter a decades long boom, particularly in terms of military production. Looking outwards, they establish new colonies, expanding towards what would be the Tortuga Dominions and the Aurigan Coalition in OTL.
As the Star League comes together, the prospect of bringing the Periphery in by force was raised and narrowly turned down. The projected expense was deemed to be directly against the principle of reducing the drain of constant military expenses. And besides, House Amaris was pro-Star League and probably only needed a little more time and a comparatively trivial financial investment to sway the Rim Worlds Republic into joining the Star League. One at a time, the other states would surely follow through.
The HAF wasn't entirely folded into the SLDF, but did hand control of their border worlds over to the SLDF First Army, which also took over all other member-state world garrison posts within one jump of the Hegemony. Gradually five further armies expanded out to similarly control both sides of all the Star League's internal borders. This didn't keep the SLDF entirely out action - skirmishes between 'bandits' along the borders took a while to die down. As this progressed, the process freed the House Armies to secure their own interiors, their periphery borders and to scale back to 'peace-time' strength.
As the economy slowly stabilized, the Star League looked for new frontiers and found them in the periphery: new colony regions were established outside the Federated Suns, Free Worlds League and Draconis Combine and opened to colonists from anywhere in the Star League. This also contributed to isolating the periphery realms from the rest of the Inner Sphere. As a result, when Gregory Amaris requested military assistance against dissidents in the Rim Worlds Republic, shortly before the end of the century, it was years before the other Periphery states were able to confirm that Star League supplies and military advisors were propping up Amaris and funding his employment of mercenaries and 'mercenaries' drawn from House armies eager to blood a new generation of young soldiers.
The result was all but inevitable: Gregory Amaris managed to live long enough to sign the admission of the Rim Worlds Republic to the Star League, although an assassin saw that it was his son who became the seventh member of the Star League Council. The SLDF's 7th Army (and associated fleet) secured the border between Lyran Commonwealth and Rim Worlds Republic - and unofficially provided a strong arm for the still shaky House Amaris.
Seeing the writing on the wall, the remaining Periphery realms realized that any internal or external conflict could be used to justify Star League intervention. Further, they were increasingly surrounded. Any spark along their borders could lead to encroachment. They forced their economies into overdrive to try to expand their own borders and increase their economic might to potentially resist SLDF invasions. In the Outworlds the weak government teetered on the edge of breakdown under the pressure of this, kept from collapse only by the outside threat.
And in the new colonial regions, colonists from rival member-states began to quarrel, leading to reaction forces from the SLDF and nearby houses to step in.
After forty years, the Star League is stronger than ever... and the peace that it promised is more fragile as well.