Here’s a Robert’s Rebellion what if.
Oberyn wanted to raise Dorne’s banners for Viserys, after Elia and her children were slain. Let’s say that Jon Arryn is killed in Dorne by Oberyn or some other vengeful dornishman.
Under the flag of negotiations.
Robert is of course, going to burn Sunspear down for this.
But let’s say that Darry does go to Dorne from the free cities. With young viserys betrothed to Arianne(who I think was born then?). Oberyn more or less intends to die fighting and spill as much Lannister and Baratheon blood as he can.
Well first off-I think BLAST remains fairly cohesive. There will likely be some minor risings in the Riverlands but Hoster will suppress any targaryen loyalists.
So both sides mobilize for war. Oberyn likely tries to get as much Essosi support as possible. The Martell brothers likely agree on a strategy of pro active defense and hoping that the targ loyalists will rise against Robert.
I can see certain reach houses deciding to back the dornish, the Tyrells are in an uncomfortable position here-Robert knows good and well that they didn’t do much except wait to see who came out on top. But they have to demonstrate their loyalty to Robert somehow. So Mace probably supports Robert. Alongside the other houses married into the Tyrell family. The Hightowers and Redwynes in particular. Rowan and other targ loyalists likely raise their banners for Viserys-and put the reach in a state of civil war.
This forces Mace to concentrate the bulk of his attention dealing with that problem.
Robert will likely wish to march through the Bone mountains, with the Royal Navy blockading Sunspear. Perhaps a coordinated offensive by sea as well. Attacking western Dorne and Starfall. The Lannister’s, Arryns, Starks, Tullies, and Baratheons will likely aim for this approach. With Tywin, Ned, and Arryn all vying for the King’s Counsel.
Numerically-they outnumber the Dornish and any reach allies they can muster.
But the Dornish decide to fight to the end. Less for Viserys and more for Elia.
They do use guerrilla attacks, assassinations, and the like to wear down Robert’s forces.
However, inevitably Sunspear will fall. And Viserys will definitely be slain. Alongside Daenerys(if she isn’t sent to the Silent Sister’s at Ned’s urging).
Assuming similar Dornish resistance to aegon and Daeron-BLAST does suffer heavy losses and many characters of canon like Ned, Jaimie, Stannis, even Robert himself could all potentially die here.
However Robert has two brothers, though Cersei will likely not be getting pregnant. Especially if Jaimie is fighting in Dorne with Robert.
If Stannis dies but Robert lives-he likely names his heir with Cersei the future prince of dragonstone, but it would be handed to a castellan. If Robert dies, Stannis takes the throne. Cersei without having given the king a son but otherwise not disgraced or having committed any treason is sent back to the west. Though I’d have to check the timeline as to when Stannis married Selyse and if he’d do so in a continuing war. I sort of doubt it.
If Stannis and Robert die-young renly takes the throne under a regency council. Though I doubt that. I can also see Ned dying, Jaimie dying, Barristan dying, to Dornish spears or poison. All of which have effects.
Ultimately I can see this securing the Baratheon dynasty or fatally weakening it. Ned will likely be more sympathetic to the Dornish though he will still fight for Robert out of loyalty. Tywin will be happy to crush the martells and Arryn will likely be aggrieved it had to come to this but not oppose the attainment or destruction of house Martell. For the security of Robert’s reign.
Basically I’m imaging the war starting more or less as soon as Dany and Viserys escaped from dragonstone-or when they were still on dragonstone. So Robert’s rebellion doesn’t actually end but the war continues.
Heck-depending on the timeframe there might not be any Cersei-Robert marriage. My memory of the timeline of RR isn’t that specific in how all the events line up. If not Tywin will still push for it as soon as the Dornish threat is dealt with.
The Greyjoys under Balon likely remain neutral to tentatively pro Robert. If the war turns into a debacle-they start raiding and reaving again. Feeling the Baratheon dynasty is going to collapse. If it steamrolls the Dornish and Robert comes out even stronger-they likely decide to wait. Balon however probably does not support Robert in any military capacity.
The crownlands are weak and pretty much tepidly pro Baratheon. Not out of any loyalty to Robert so much as they know they will be crushed and replaced if they don’t support the king.
In any event, LF likely never reaches the small council. And Varys has to dramatically change his plans. Depending on how strong or weak the Baratheon dynasty comes out of the war.
It may be more houses rise for Viserys if he is in Westeros to rise for. More Riverlands and reach houses, potentially some lords of the crownlands and narrow sea. If Robert doesn’t have a chance to solidify his rule.
The best Viserys can hope for is that these scattered pro targ rebellions buy time and weaken Robert enough for the BLAST coalition to fracture. But honestly I don’t see there being some massive “for the dragons!” uprising against Robert. Due to the fact Robert has smashed them in battle, and his dynasty has at least five great houses behind him.
@Ganurath thoughts here?