1 April, 1081 (The Alexiad), Serene Doge Gerardo I Visconti, Serene Doge of Pisa
Link
Just in case you guys might be interested, some three months before the quoted post was made (just before the release of the Republic DLC) I started an AAR with a Ruler Designed replacement for that doge in 1 January 1078, which is when he succeeded the previous della Gherardesca doge. I've had some very interesting adventures so far, though mostly playing like a feudal ruler with city vassals and more cash. My dynasty tended to have 1-2 adult males for like 100 years.
I would also like to suggest Saint Louis IX. You start in 1226 with a child king, and England has de iure ducal claims in Aquitaine. My first time through I had Henry III in my jail.
In any case, starting so young means that you'll still be relatively young when your Short Reign wears off. You will have Long Reign bonuses when things get really tough. Jerusalem is on the verge of collapse and can go down in two holy wars, while the queen is the Empress of the HRE. Both BYZ and Latin Empire are threatened by Rum, though Byzzies are surprisingly tough. Trebisond will probably soon cease to exist, and Georgia is very likely to follow suit, as is Armenia Minor. Also Antioch and Tripoli (ruled by brothers, but there seems to be more claim infighting than alliance) are exposed to holy wars from the neighbouring emirates of Aleppo and Edessa, all of which are ruled by descendants of Saladin (the Emir of Edessa is actually a son of Saladin), which means they are of the same dynasty as the Sultan of Egypt.
However, like in history, Egypt is unstable, and there are often civil wars. This means you can hit for a couple of duchies within a shorter time-frame than you normally would with truces with the sultan. Also, sultans have a shorter life expectancy in the circumstances, occasionally ending your truces. These days Nubia has been removed from de iure k_Egypt, which means that k_Egypt contains: d_Damietta (4 provinces in the Delta), d_Alexandria (5 provinces in the west), Cairo (2 provinces south of the Delta), Aswan (3 provinces at the south end), d_Sinai (4 provinces). The Delta has good supply limits, as does Cairo, while d_Sinai and d_Alexandria are undeveloped with few holdings, which means they're relatively easy to capture. All of Egypt is 18 provinces, of which you need 10 (51% rounded up to full province) to usurp Egypt. You can win the holy wars for the three coastal duchies without needing to incur desert penalties to supply, although my favourite marine stomps (funnelling armies into and out of battles with ships) has been nerfed with longer loading/unloading times and halved morale on ships. Usurping Egypt makes all the emirs independent as opposed to being united under one Sultan.
You have some interesting opportunities because the Pope will likely soon call a crusade to restore k_Jerusalem to its full borders, while he might alternatively or additionally call a crusade against Egypt. The Mongols, whom you will likely face eventually, are beatable. You can see some interesting stuff if you goad them to attack you in the mountains of Georgia, where you will have like 15-20K supply due to province religion and culture. You can use your Unyielding commanders, Flankers, Defenders, Holy Warriors, Flankers (unlike Inspiring Leader for a fixed 10% at the centre, Flanker does scale with Martial ability).
Your cultural retinue is knights, by the way, and you get to choose the education for the king (who should have about 3 traits at start, not necessarily positive ones, they're random anyway), after which you can use him as a commander. I did before they nerfed single flanks, which allowed me to pick up Inspiring Leader and Cav Commander. So I used him to lead cultural knights packed into one flank etc.
My next saint in the AAR will be David I of Scotland, starting in April/May 1124 as a 40 year old King of Scotland, although there are closer heirs (sons of elder brothers). He should be available from about 1113 as Duke of Lothian (real life: Prince of the Cumbrians, i.e. Welsh settlers/leftovers/Alt Clud remnants etc.) Son of St. Margaret of Wessex, nephew of Eadgar. Supported Empress Mathilda (his niece) against King Stephen, invaded England and lost. His great regret was being unable to prevent the atrocities committed by his savage armies (and perhaps the entire invasion itself). He supported the Church and introduced reforms, becoming known as the 'King who made Scotland'. Unfortunately, the mechanics of the game will likely prevent him from getting any significant piety or even building bishoprics. However, he will probably be somewhat likely to have an opportunity to deal with the Norse or any independents in de iure Scotland or perhaps some adventurers etc. However, it's also likely that he will die of natural causes several years after your start.
I'm also planning to look at
Charles the Good of Flanders, who is actually Danish and son of (Saint!) Canute, the outside bet in 1066 (his invasion of England didn't come to be), whom you seem to have already listed (judging by the Wiki page).
You may want to look at
Guy de Lusignan (King of Jerusalem 1186), as well as
Sybilla,
Isabelle and
Melisande. I'm tempted to start an AAR dealing with the women on the throne of k_Jerusalem. Please note that the game does not follow history in 100%, so you may need to trace the rulers month by month manually. They may also take turns with each other in being listed as the king/queen of the time.
Another interesting character is
Boleslaw III Wrymouth of Poland, who should be on the map in 1109 as the sole ruler and in any case should be on the map in some capacity since 1102. He has an elder 'bastard' brother Zbigniew, but Zbigniew's supposed illegitimacy (supposedly something like handfast marriage but in any case missing the religious form that was expected at the time) is controversial. Boleslaw should have a complete k_Poland. Historically, he struggled against the HRE (but was respected by them) and expanded into Pomerania. Notably from the point of view of CK2, he
instituted Seniority Succession when he died in 1138. Unlike the pretty shape in the game, Poland was really torn apart until 1296 (a short-lived king with limited territorial rule), then came Bohemian conquest, and then in 1306 the land finally was somewhat united by Wladyslaw the Elbow-High (which I tried to reenact in my Dominus Regni Poloniae AAR). However, large parts of the country were missing, and not even Casimir the Great (the king in the last start, his son) managed to get the full k_Poland back, although he did expand into Ruthenia. It can be said that Poland never recovered from the consequences of that Succession Statute. In fact, modern (post-1945) Poland looks sort of like 1138 Poland, and the de iure kingdom/duchy makeup of Poland is basically based on that statute. Boleslaw (Boleslaus) is the son of the Duke of Masovia from the 1066 start and nephew of the king from that start. Also, the sond of Boleslaw the Bold should be alive and have a claim.