I don't know if any of these have been mention yet, but here are some things I would like to see:
1) The ability to add a greater variety of titles to the same tier, if only for purely immersion reasons. For example: at Kingdom tier: Grand Duchy/Arch Duchy, Grand Prince, Prince (for Principalities, such as Principality of Antioch).
2) This kind of goes with #1, the ability to make the naming of tiles more dynamic in game, based on culture, hierarchical position, number of titles etc. Being able to set how title ranks are named based on certain criteria would be welcome, such as a duke being called grand duke if he/she owns multiple duchies, but is not a king, being called High King for owning multiple king titles but not an emperor, and having principalities form by granting your heir a title.
3) Finally, being able to have your tile name change based on councilor position, and being able to add different councilor positions based on title tier. For example, having Duke example called Marshal example, and having the marshal of a King/Emperor called grand marshal.