I know it's selfish, but I would like a bit of Irish flavour. Polygamy, trial marriages, blinding, random defections, hostages, fosterage/education bonuses, the ability to join any factions inside the de jure kingdom of Ireland, regardless of whether or not it is formed, titles claims for all grandsons of the current king, no female inheritance or female line inheritance, funky inauguration ceremonies, interfere with subordinate inheritance (with penalty), depose subordinates to replace them with more favourable candidates of the same dynasty (with penalty) church dynasties, tánaisde where you nominate your heir before death, though it does not guarantee him succession due to (almost) automatic faction fighting, raiding. Would also be interested in an ability to partition titles to keep your vassals to busy fighting themselves to start on you.