I'd be willing to take a back seat role in all this. Maybe write up some pedia entries or give advice if need be.
Some things to consider:
Techs would be interesting to implement. I could imagine just off the top of my head a tech like Wildfire(for King's landing, which would allow a wildfire- launching artillery unit), Obsidian Weapons(for the Nights Watch and maybe Stannis' army as well), Religion of the Lord of Light( a government type), and maybe a tech that allows Daenerys' dragons to upgrade into bigger versions of themselves. This might seem terribley specific, but I know in EFZI 2 that there are specific tech trees for different factions. I never knew how it was done, but a similar thing might work here.
It should probably be worked out beforehand which time period we do, or if we just do an overall scenario, or even multiple scenarios depicting different points in time(something like Rise and Fall of the Roman Empire).
I think most of the units we need are already made. The only thing I can think of is a Giant riding a Mammoth, but even that we could simply split up into 2 seperate units. White Walkers also might need making.
I highly recommend using mass regicide(Stannis, Melisandre, Davos; Cersei, Tyrion, Joffrey; Robb, Caitlyn, Bran; Daenerys, Dragons 1,2,3...) King units could get special bombard bonuses, and dragons could eventually get really high lethal bombard, etc. This might work better in an epic multiplayer game.
We could simply make Daenerys a treasure unit, and King's Landing a VP location, and if anyone(herself, Dorne, Iron Islanders...) takes her to the Iron throne, they win.
Anyways, just some thoughts.