The guilds from the Rhine tribunal.
The system of having magical political parties really ought to be core. It is a brilliant way to inform the political stance of a magus and the fact that magi choose to join guilds based on their personal convictions rather than their house adds a lot of depth to characters and what can be done with them.
Contrast e.g. membership in house Jerbiton versus membership in the apple guild. A member of house Jerbiton will be expected to favor mundane integration because their pater told them to. A member of the apple guild favors mundane integration because that what they believe in.
Having guilds as a core system will allow the houses to be more like lineages and societies without including personality types.