Crusader Kings 2 was fantastic, and a bit more realistic than its predecessor too. Only bug i spotted was that during a campaign with Barcelona, the game would end abrruptly, even though my leader was healthy and i had an heir. I could resume the game from the annual autosave but it got kind of irritating.
Also would be fun to be able to create titles more freely, CK1 style. (I reigned over the whole western part of europe as king of ireland, would be fun to make an irish empire

)