The hordes look promising!
One inherent challange with fitting the hordes into the new idea system, is that they generally didn't develop and increase their horde skillz during the game timeframe, but instead stagnated and declined. One way to approach this could be to flip the NI system upside-down for the hordes. They start the game with all their NI bonuses, and have National Ideas that are negative, and slowly negates their starting bonuses. This would mean that the hordes are at their strongest when the game starts, and then declinces. That is, they decline IF they buy other ideas. So, it sets up a choice between sticking with the traditional ways, and remain very strong military wise, or skipping the old ways and lose their military might gradually.