If you're still doing idea groups I have a couple of suggestions
Firstly, Georgia.
It starts off in a particularly interesting position surrounded by invading hordes, and can be pretty fun to play as, so it's a bit disappointing that they don't get their own idea groups in vanilla, given how unique the state was throughout history.
The ideas could mostly focus on Georgia's defence against the hordes and give bonuses to things like fort defence, morale and land leader manoeuvre (for strategically manoeuvring around the mountains and stuff I guess, although I'm not sure what this actually does in the game). You can read about Queen Tamar of Georgia (
http://en.wikipedia.org/wiki/Tamar_of_Georgia though she is a bit before the time frame of the game, it's still important in georgian history) and the Georgian system of feudalism (
http://en.wikipedia.org/wiki/Georgian_feudalism). The feudalism could be sort of represented in-game through a bonus to vassal income. Queen Tamar's golden age could be represented through yearly prestige and legitimacy bonuses (these would probably have to be the traditions / starting ideas because Queen Tamar's reign was 250-ish years before the game's start), and the expansion into the neighbouring muslim states could be represented with some bonuses to holy wars (I think it's possible to give a manpower bonus during holy wars). Also the system of serfdom in Georgia was quite rigid, which could manifest in a bonus to production efficiency