Multi-tiered titles: the ability to grant a higher title to a character with a lower title and have it replace the lower title, e.g. a Duke title replacing a Lord title. This is in addition to complementary titles like Warden of the Cinque Ports, Lord Chancellor, etc.

Also, the ability to grant titles to your heirs. It always aggrevated me in MTW that your often most-capable characters couldn't run provinces. When a king died and the previous heirs became regular lords, that was always a great day. :)