Great to see there's interest on this!
I do believe Robert's Rebellion would be complicated since it would have to prevent Tywin and others in order to achieve some balance, which is something that would not happen with The Lion and the Rose (with mostly formed fields) and A Touch of Summer (that resembles a sandbox in most aspects).
I also think making the White Walkers the chief threat will make it hard to make good antagonists of playable characters which was easily my favorite part of the previous game Willas, Roose, Littlefinger, Danaerys, and arguably Tywin and Renly were excellent antagonists in the previous GoT game, imp. Who can fill these shoes? Especially if any villains would be blank slate characters. That said, if most folks want to do this scenario, I'm game!
That could be a problem, although on the other hand introducing new characters would help the players create their own personalities and engage in a different style of roleplay. And if my initial thoughts end up in the final game we will still have Tyrion, Myrcella, Trystane, Margaery, Sansa/Arya and Asha to take the mantle of the fallen schemers.
I'd personally like The Lion and the Rose, but keep in mind that Joffrey would not have personal rule, since a regent rules for his or her monarch until they reach their sixteenth name-day, so I would suggest killing off both Cersei and Tywin for maximum chaos.
That could work, and it poses an interesting fight between Tyrion, Kevan, Mace and other powerful Lords.
Assuming the finalists are "A Touch of Summer" and "The Lion and the Rose", I think the playable characters might go like this, give or take a few:
The Lion and The Rose:Main:Joffrey Baratheon, King in the Iron Throne
Daenerys Targaryen, Queen of Meereen
Stannis Baratheon, King in the Narrow Sea
Roose Bolton, Warden of the North
Petyr Baelish, Lord Protector of the Vale and Lord Paramount of the Trident
Tyrion Lannister, Lord of Casterly Rock
Mace Tyrell, Lord of Highgarden
Doran Martell, Prince of Dorne
Spoiler:
Aegon VI Targaryen, claimant to the Iron ThroneOptional:Ser Brynden Tully, the Blackfish
Walder Frey, Lord of the Twins
Lord Varys, Master of Whisperers
Jon Snow, Brother of the Night's Watch (right after the battle of the Wall)
Wyman Manderly, Lord of White Harbor
Some of the Kingsmoot players (Victarion, Euron, Asha, etc)
A Touch of Summer:Main:Daemon II Blackfyre, King of the Dragonlands
Asha Greyjoy, Queen of the Iron Islands
Robb Stark, King in the North
Myrcella Arryn, Queen Consort of the Vale (King Robert incapacitated due to madness)
Tyrion Lannister, King of the Westerlands
Prince Steffon Baratheon, Lord Commander of the Commonwealth Army
Shireen Baratheon, Queen of the Stormlands and head of House Baratheon
Aerion Brightflame, New Valyrian Emperor
Antarys Tarquinia, Sealord of the Most Serene Republic of Braavos
Trystane Martell, Prince of Dorne
Optional:Jason Mallister, Lord Regent of the Riverlands
Mathis Rowan, Lord Regent of the Reach
Mance Rayder the Old, King of the Wall
Utheros, High Septon of the Faith of the Seven
Arginel, unofficial leader of the Faith of Oldtown