IMO, the Eyrie can be unexpugnable without making it unfair: it's just 1 SC and you can control SeagullPort, The Fingers, and everithing around it without further problems. Storm's end can be taken if attacked by sea + land. The riverlands won't necesarily have to be neutral, there are several games with a central power, similar to germany in classic. About Essos, it depends on how much you want to expand the map, but theoretically you can take it as fas as Asshai.
PS: i don't have a clue of what "the works" are, if you can enlighten me... And also, if wanted, i can make the map and that, but i don't know how to code (i can be taught to, however)