Here's another idea: What if certain early authors' hateboner for the Cappies didn't interfere with in-universe logic, and in the 4th Succession War the FedCom power actually go for the
logical target: The Combine. Not only is that their mutual enemy, with centuries of loathing behind that enemyship, much more than what the realms feel for FWL or CC, respectively, it also is a far more logical choice if you want to conquer a corridor between the realms. Hell, in canon, the Terran Corridor only happened by accident when Ridzik's new "Tikonov Republic" decided to attack the FWL for shits and giggles. So instead, FS and LC decide to create a Dieron corridor, targetting Proserpina, Al Na'ir, Dieron, Altair, Kessel, Vega.
That as a general question: Both LC and FS focus on the DC in the 4SW, to open up a Dieron Corridor.
Now, an idea for a scenario I had based on that:
There is no canon 4SW miracle logistics. The war happens at the same glacial speed as the previous Succession Wars. Really, having the 4SW over in 4 years, with half a successor state conquered, while the previous wars had lasted decades, is just ridiculous. So, LCAF and AFFS slowly push the Combine back, but are taking losses from the CC and the FWL. Especially as the Taurians also join in the fray to attack the FS, and Michael Hasek, conspiring with Maximilian Liao, uses this as pretext to build up an own zone of authority, basically sabotaging the Capellan front.
Things eventually go sour for FWL and CC as well, though. Maximilian Liao dies, and using the characterization from the old Liao Housebook,
both Candace and Romano are cold, power hungry bitches - and Romano, to boot, with a murderboner for the FWL. With the aid of Maximilian's widow (who is opposed to Candace because Candace protects Tormano, the third sibling - I swear it's all soap opera with those Liaos) she becomes the new Chancellor and since the FS front is more or less taken care of attacks the FWL.
This causes Andurien to throw a tantrum about that open failure of the Concord of Kapteyn (the loose, never really realized alliance between CC, FWL and DC in response to FedCom). With the aid of Andurien, which invades FWL border worlds, Andurien secedes, though only nabs some surrounding FWL and CC worlds instead of going on a full out offensive. The end result of that little shuffle is that Canopus keeps the conquered planets, Andurien is independent and allies (but not united) with Canopus, FWL and CC return to a truce. Candace ends up Chancellor, and Thomas Marik Captain-General, and just like in canon he can use the Andurien secession to push through his centralization efforts. Overall, the FWL still has made some few gains against the LC, and the CC some considerable gains against the FS thanks to Michael Hasek (who is eventually deposed by his son George and flees into the CC).
Meanwhile, the DC is gutted, despite ever increasing covert ComStar support. As in canon, Theodore Kurita becomes Kanrei and hence de facto ruler, but there is only so much he can do. Deciding a grand gesture is necessary, Coordinator Takashi Kurita, his father, commits seppuku, which throws the Draconian society into such a shock that Theodore as new Coordinator has basically free hand for even the most radical reforms. He seets Raselhague free (smaller than the canon FRR) to create a buffer to the LC and to concentrate on Dieron. However, as by that time FWL and CC had their falling out, the Draconian counter-offensive mainly fizzles against FedCom reinforcements
However, by that time it's somewhen after 3040s already. This was really not what Archon Katrina had in mind when she had issued her
Peace Proclamation. The Davions would like to push the issue; the Lyrans really, really wouldn't. They have their corridor, trade can flow freely between Tharkad and New Avalon now, and they're thus set to dominate the Inner Sphere economically within a generation or so. War is just wasteful. That's the Lyran way
In addition, there is the issue of Hanse's and Melissia's marriage. It's a political one; it's silly to turn it into something to do with love as canon did. And the increasing political disagreement between Tharkad and New Avalon puts a strain on the marriage. Also, who will administer the Dieron territories, anyway? So, to solve all those issues, the conquered Dieron worlds are formed into a Principality of Dieron, with Melissa as Princess. She holds court in Dieron, and takes her children (or maybe all her children
except Victor as the eldest) there. Per the plan for the future, as soon as Katrina, Hanse and Melissa have all died or abdicated, Victor will unite the three realms and hold his court at Dieron, which is neither Steiner nor Davion and hence a good compromise.
To end the war, there is an actual peace conference, instead of it just petering out like the 3rd. The DC has to accept the current borders, but gets to live. LC and FRR exchange some worlds, but Tamar getting decisively less out of the deal as it had wanted (as in canon, but less severely so). FS and CC also recognize the current borders. LC and FWL, the two most trade orientated and most peaceful successor states (really, a shame they had to duke it out with each other for centuries..) actually hash out a much more comprehensive agreement, which includes the establishment of two buffer zones, as both sides honestly want to avoid war now. Also, DC, CC and FWL try to reinstate the Concord of Kapteyn.
So, by dreaded 3050:
Political
-The DC is still gutted, and the smaller FRR even more of a joke than its canon counterpart.
-The CC is slightly larger than pre-war (to say nothing of canon post-war), having made gains especially in its "southern" half, and the FWL is about even.
-FedCom exists, but there is a third component to it. Also, New Avalon is frequently at odds with Tharkad and Dieron (Katrina and Melissa usually side with each other, and most of the Dieron administration is made up of Lyrans) - it is more of a political, pragmatical alliance here than a literal marriage of love.
-Two buffer zones between LC and FWL; one of them a sort of mini-Chaos March and one a functioning mini-state including the industrialized worlds of Gienah, Ford, Loric. Let's say the Wolf Dragoons use that instead of Outreach for their base of operations...
-Andurien is independent and allied with Canopus.
-The Taurians are again in open enemyship to the Suns, and likely to ally with the Capellans (though no Triple Alliance as in canon, as Canopians and Capellans are on opposite sides)
Personal
-Candace Liao is Chancellor of the CC. And not married to any silly Allards, so no Kai Allard-Liao. In the wheeling and dealing that made her Romano's successor, Romano actually gained the Prefectureship of Capella, so she is still around as well. But not quite as insane as in canon. Since Romano's lover was a random Maskirovka agent, Sun-Tzu and Kali Liao may still be born, but may have a bit of an healther upbringing.
-Hell if I know if the FWL is led by the real or like canon a fake Thomas Marik. The whole WoB "Master" arch always seemed very silly to me. But with Andurien's secession never really disputed due to the needs of other fronts, Duncan Marik could still be alive. Probably in exile somewhere... Dieron or New Avalon. Always good to have a spare Marik on your side
-Dieron could also be an exile court for some losers in Lyran power struggles. Say Frederick Steiner, also known in canon as Anastasius Focht
-And speaking of exiles, Michael Hasek is also still around spins schemes on Sian, though his agent network is no match for the Maskirovka.
-On Canopus, Kyalla Centralla's course has been completely vindicated. Therefore, she won't be toppled by her daughter Emma, and will most likely move Canopian culture away from being a pleasure circuit towards somewhat more militarism.
-If we say Victor stayed on New Avalon, then unlike canon, the association of Victor <-> FS and Katherine <-> Lyran actually makes sense. He grew up at a Feddie court, and she and the others at a Lyran court. So there could be genuine cultural differences here that would allow making Katherine more than just the most generic of evil cackling villains.