If William I of Sicily dies before his father....

[Zioneer]

Then who are likely contenders for the Sicilian throne, since all of Roger II's legitimate sons are dead? Would either Simon of Taranto (Roger II's illegitimate son) and/or Tancred of Lecce (Roger III's illegitimate son) be as the new king?

Or would one of Roger II's daughters and their husbands be the preferred claimant to the throne? Wiki says that this is a list of Roger's daughters and their husbands:

-A daughter, wife of Rodrigo Garcés (later Henry, Count of Montescaglioso)

-A daughter, wife of the neapolitan nobleman Adam;

-Clenenza, married Hugues II, Count of Molise;

-Adelisa (d. aft. 1184/87) married firstly Joscelin, Count of Loreto, and secondly Robert of Bassonville, Count of Loritello;

-Marina, married the great admiral Margaritus of Brindisi.

Some of these husbands might not be married to the daughters by the time of Roger II's death in 1154.

Or would an entirely new dynasty arise? Could any of the barons make a valid claim for the kingship? Or a foreign monarch, invited at the behest of the barons?

 

[Yanez de Gomera]

I would say that, whoever becomes king, civil war is quite probable: a civil war that could lead to the Byzantine recovering Apulia or imposing a friendly candidate.

I think that the most likely candidates are Simon, Tancredi and Robert Bassonville, count of Loritello (consider that all survuving daughters were illegitimate too).

Much changes with the moment of William's death: if there is enough of a gap between it and Roger's death I think that the king could manage to push for Simon as his successor, maybe even managing to have him receive some sort of papal recognition as an legitimated son. This would obviously put him in a stronger position that Tancredi and the County of Loritello, but still I think that the risk of civil war is very high.

 

[Janprimus]

I think you're forgetting two other important powers on the Italian Peninsula, the Papacy and the ((Holy) Roman) Empire. They too will get involved in the following power struggle. However I'm sceptic that the Byzantines are powerful enough to take Apulia. Southern Italy was also claimed by the ((Holy) Roman) Empire and the Papacy has stakes here too. After all Roger II had been invested as King of Sicily, Duke of Apulia and Prince of Capua by the Pope (after being acknowledged as such by an Anti-Pope first).
It's far more likely that the Sicilians will use one against the other; accepting a nominal position as Papal Fief (or maybe even Imperial Fief, though that's far less likely, especially since the Papacy won't like that) are preferable over having to give up lands (to in this case the Byzantines).

 

[Carp]

The Byzantine invasion in 1155-6 did take Apulia, or at least the coastal part, before the death of their commander, the incompetence of his lieutenants, and divisions among the Greeks and rebels caused the campaign to fall apart. I too have my doubts as to whether the Byzantines could have accomplished any permanent restoration of the Italian catepanate, but a peace favorable to the Byzantines or a decisive intervention in a pre-existing Sicilian civil war is entirely plausible.

Simon of Taranto and Tancred of Lecce seem like the best candidates to me. Emperor Frederick can do it, but only if things go much more favorably for him ITTL; IOTL he was so stymied by Papal and Lombard opposition in his quest for dominance in Italy that he was never really able to mount a serious invasion of the south. Manuel Komnenos made a better go of it than Frederick ever did. I can see Norman barons inviting in Frederick, because some of them did exactly that historically, but it doesn't really matter all that much if Frederick can't actually get down there with an army.

 

[Zioneer]

I think you're forgetting two other important powers on the Italian Peninsula, the Papacy and the ((Holy) Roman) Empire. They too will get involved in the following power struggle. However I'm sceptic that the Byzantines are powerful enough to take Apulia. Southern Italy was also claimed by the ((Holy) Roman) Empire and the Papacy has stakes here too. After all Roger II had been invested as King of Sicily, Duke of Apulia and Prince of Capua by the Pope (after being acknowledged as such by an Anti-Pope first).
It's far more likely that the Sicilians will use one against the other; accepting a nominal position as Papal Fief (or maybe even Imperial Fief, though that's far less likely, especially since the Papacy won't like that) are preferable over having to give up lands (to in this case the Byzantines).

Could different claimants become the de facto claimants of each side? That is, say, Tancred could become a pro-Papal candidate, Simon the pro-Greek candidate, and Robert Bassonville the pro-Imperial candidate? Obviously it probably wouldn't be as clear-cut as all that, but could those factions emerge as the de facto factions for the Sicilian throne?

I would guess that each claimant would have some kind of "powerbase" during the Civil War; Simon's powerbase would probably be Taranto, while Robert Bassonville would likely be most powerful among the northern-most barons on the mainland. I wonder if Tancred would be popular in Sicily proper?

The Byzantine invasion in 1155-6 did take Apulia, or at least the coastal part, before the death of their commander, the incompetence of his lieutenants, and divisions among the Greeks and rebels caused the campaign to fall apart. I too have my doubts as to whether the Byzantines could have accomplished any permanent restoration of the Italian catepanate, but a peace favorable to the Byzantines or a decisive intervention in a pre-existing Sicilian civil war is entirely plausible.

Simon of Taranto and Tancred of Lecce seem like the best candidates to me. Emperor Frederick can do it, but only if things go much more favorably for him ITTL; IOTL he was so stymied by Papal and Lombard opposition in his quest for dominance in Italy that he was never really able to mount a serious invasion of the south. Manuel Komnenos made a better go of it than Frederick ever did. I can see Norman barons inviting in Frederick, because some of them did exactly that historically, but it doesn't really matter all that much if Frederick can't actually get down there with an army.

I agree with your initial assessment of the Byzantine campaign, they definitely can't keep it all. I wonder if they could extract a peace treaty out of a successful pro-Byzantine claimant, and other concessions?

I'm more favorable towards using Simon of Taranto or Tancred of Lecce for the purposes of the TL (which will be a reboot of my old Norman Sicily TL), so I'm wondering the advantages of each. For story purposes, Simon is much more of a blank slate, so I can theoretically use him as I see fit, whereas much more is known about Tancred, but his competence and personality is known, so I know it's plausible for him to keep things together, as he did in a much more desperate situation.

And as you say, Emperor Frederick and Manuel Komnenos might make an effort to conquer parts of Sicily or all of it, and put some kind of viceroy in charge. I do wonder what other neighboring powers could meddle in this civil war, and who they might favor?

 

[Zioneer]

A few side questions:

-How old were Simon of Taranto and Tancred of Lecce in 1154, respectively? I read somewhere that Tancred was in his twenties at the time of the rebellion that nearly toppled William I, but looking through John Julius Norwich's books on Sicily, I can't find anything confirming that. The book says that Tancred died in "early middle age", which can mean a lot of things. A few dubious Internet sites claim Tancred was born in 1138, which would make him about 16 at the time of Roger II's death. There is no mention of Simon's age. Obviously both were old enough to take part in actual physical violence by the time of the rebellion, and Simon was made Prince of Taranto in 1144, which means he was probably at least old enough to administer that territory. In my old TL, I randomly gave Simon the age of 28 in 1154, I think on the logic that he would probably be near to the same age as one of the legitimate sons. Should I just make up the ages for the sake of the TL?

-Assuming either Tancred or Simon triumph, who are some eligible noblewomen at this time, either in Sicily itself, or in a neighboring or somewhat close kingdom/duchy/etc? It doesn't appear that either Tancred or Simon were married at the time of the rebellion against William I.

-Tensions only seem to have grown to a boiling point between the Latins (Catholics), the waning Greek Orthodox, and the Muslims by the time of the rebellion against William I, so I imagine that neither Simon or Tancred would see much use in bashing the Muslims as their ticket to power. Most of the civil service seems to have been Muslim or Orthodox, anyway.

-Lastly, I may have asked this in threads past, but could or would King Tancred/Simon cultivate a military force which does not rely on the Norman cavalry and thus the Norman barons? It seems like the barons are more trouble than they're worth militarily. Could a more active king than the two Williams reform the military somehow, or even create a force similar to the Varangian Guard?