I have just started reading Hedge Knight, so I could get inspired to create a scenario 100 years earlier. the trouble is besides some interesting characters the CK political situation is less interesting than Roberts Rebellion as the civil war would only be between Targaryens. In CK gameplay terms it doesn’t really matter which Targaryen is the victor.
I was a bit disappointed that despite general improvements in the 300AL Scenario the AI hardly ever seems to get it together on ANY front (Stannis fizzles; despite hefty troop event bonuses and more land the Ironborn still get raped; Dany does nothing even with OP Dragon events, Tommen doesn’t do much; neither do the Others or Wildlings.
I also considered making a scenario set just before Aegon’s landing. Trouble is the lack of a 4th tier really limits things. Aegon conquered the existing Seven Kingdoms who he made vassals (he was effectively emperor… in CK1 this is not possible). I could make him ILKH/GOLD Mongol but then he will hold all of Westeros as his personal demense. Will have to wait for CK2.
The only other options are:
1) Scenario set during War of the 5 Kings. Joffrey

, Rob King of the North and Riverlands, Renly controlling the reach and Stormlands. Some juicy land grabs / claims and conquered kingdoms should be possible (even by the ai). As its only a couple of years earlier than the 300AL scenario I wont have to do to much reworking of the family trees (although there were a lot of deaths esp around Riverlands). Would be fun to play Viserys as an un-historical county near Pentos and try claw his way back to the Iron Throne.
2) Set well before Aegon’s landing, maybe time of Heroes, or when there were Kings of Reach (Gardener), King of Riverlands, etc Trouble with this is there are no family trees going back this far. I would have to make up 99% of it all myself. If I am going to do that I would rather make an entire new map/world/mod myself, moreover one which meshes will with the gameplay mechanics of CK, which is what I am doing atm. The map is 100% complete and looks VERY nice (not so crowded and narrow as Westeros one). I have started the main Kings family trees going back 10 generations.. but is a lot of work.
Let me know if people are keen for 5 Kings Scenario
Very, very keen on a War of the Five Kings Scenario

Also a few things I noticed and some suggestions.
-Supposing the present Arya Stark is the impostor(considering her location), she should be married to Ramsay
-Ramsay should have the surname Bolton instead of Snow in the later scenario
-(Perhaps Bran Stark could be alive somewhere, a wildling area perhaps or a friendly lordship)
-Stannis goes pretty well in my games with R'hllor as his faith, so I think it will do him no harm to change his faith from "catholic"(seven) to "pagan"(r'hllor)
-Loras could use that severely injured trait, considering he was boiled, that would give a fair chance of either living or dying
-The Queen of Thorns is strangely dead in the latest scenario, also a boost in intrigue could be nice for her
-(Isn't Littlefinger's father dead already?)
-Daven Lannister became Warden of the West instead of Kevan, the latter should still have Lannisport though
-I think Lancel Lannister could go to the Warrior's Sons and gain the "bastard/divested" and "celibate" traits, some minor lord could replace him or his lord father
-Many houses who joined Stannis had already lost their holdings by the time he goes to the wall and went with him there. The Florents for example should all be in his court while their lands should be ruled by Garlan Tyrell (there are some others too, like the Meadows)
-If I am not mistaken Stannis left Elwood Meadows to watch for Storm's End
-The Karstarks could start as vassals of Stannis
-Cotter Pyke (Night's Watch) is an Ironborn...don't know his faith though
-Ottyn Wythers died in the Fist
-Craster died, maybe make one of his wives "lady".
-Brienne of Tarth could be dead (even though I like her)
-Perhaps add a Lordship for the Brotherhood Without Banners, with Dondarrion, Catelyn, Thoros etc (tricky though)
-Jorah Mormont should be a Tourney Champion
-Perhaps the Clansmen and Wildlings could follow the Old Gods (orthodox) while the "jewish" tag could be used for either worship of the Others or some eastern faith like the Many Faced God