Maritime and Naval are lacking, even if that has as much to do with Naval combat vs. AI in general. I'd combine the two, in addition to/or add some ideas that aid on the land side of things, like reducing the penalties for landing, or letting ships act as Supply Depots, increasing the supply and reinforcement rate of friendly regiments on the coast.
Administration I would like to see get broken into two; one a military idea focused on Mercenaries, and the other and actual administration idea. The core cost reduction would either need to be removed (which would mean a level of howling that would make the territory change look like a polite criticism) or more likely, similar effects seen in other ideas, especially ADM ones, so that it's not the must-have mana saver.
Finally, I'd like to see some ideas focused on Estates, now that they are part of the base game. For example, Religious might increase the base influence of the Clergy and give it a new interaction, while Aristocratic might increase the base loyalty of the Nobility, making it easier to make demands or eat loyalty hits from events. Maybe even an idea that reduces the base autonomy of provinces that are controlled by an estate.