WI: John I of Aragon had a son

Assuming James survives (making his successor James III of Aragon).

First, it's possible that Sicily and Aragon would remain separate realms. Martin was crowned while his brother was still alive, so we can safely say this separation was planned from the start.
I wouldn't rule out a Majorca-like unification or dynastic failure from Martin descendants, as IOTL, but Sicily wasn't seen at first as a subordinated crown as Majorca technically was. Critically in the latter situation, I could see the House of Urgell (it could have "interesting" consequences, revolt, exchange of territories, etc) or even Angevins taking Sicily (critically if a similar revolt than in 90's takes place).
But there's a real possibility to have Sicily as a client kingdom of Aragon, if not entierly independent.

Then, assuming James III have himself offsrpings, or that Martin is still alive or have enough childs himself, we could avoid Aragonese Interregnum. Meaning maybe a stronger royal authority above the assemblies, not needed to resolve this conflict (it's always a bad idea asking them to decide who would be king), a lesser Castillan influence on the peninsula.
It could help limiting bandositas, but Peter IV's costly reign made hard avoiding them entierly.

Probably no support for Avignon's pope, or not that certain at least, without the familial relation (The pope being a relative of his wife)

Maybe a more western Meditteranean policy, with Aragon actually taking on outposts and harbours in North Africa. I wouldn't see Sardinia being that succssful in its rebellion, but maybe whoever controls Sicily could take it from himself (that included an Aragonese king of Sicily of course).

So, maybe a stabler, but less hegemonic Aragon?
 
I realize that this would be further down the line, but would a surviving house of Aragon (rather than a side-line of the house of Trastamara) be equally/as interested in the unification of Spain a la Ferdinand and Isabella? Or would/could it remain as two separate kingdoms?
 
I'm not quite sure.

You always had some sort of medieval variant of "Iberism" since the Early Middle Ages, so I think the idea itself is gonna survive the PoD. Critically if Aragon is weakened enough at some point, and less powerful than IOTL being a plausible outcome (depending how succession of Sicily goes), you could see tentatives of unifications.
After all Castille didn't tried anything else than dynastic union with Aragon after Martin I's death, so the tendency is there.

Now, with a different house, and maybe a still present rivality between France and Aragon, with an existing Franco-Castillan alliance, you could have room for more tensed relations, making Aragon some sort of Eastern Portugal (actual tentatives of political union, all of them failing).

IMHO : you wouldn't get rid of the unification as a political goal, but you'd have more room to prevent it.
 
Aragon's salic-ish law of succession makes it a little safer than Portugal from being inherited by a Castilian monarch as long as the dynasty survives.

But, of course, a maiden regnant queen/crown princess of a neighboring kingdom will always be yummy. Should the opportunity arise, the King of Aragon won't think twice so... yeah, regardless of its dynasty, Aragon is indeed still at risk of being absorbed by Castile via the OTL route.

Assuming James survives (making his successor James III of Aragon).

First, it's possible that Sicily and Aragon would remain separate realms. Martin was crowned while his brother was still alive, so we can safely say this separation was planned from the start.
I wouldn't rule out a Majorca-like unification or dynastic failure from Martin descendants, as IOTL, but Sicily wasn't seen at first as a subordinated crown as Majorca technically was. Critically in the latter situation, I could see the House of Urgell (it could have "interesting" consequences, revolt, exchange of territories, etc) or even Angevins taking Sicily (critically if a similar revolt than in 90's takes place).
But there's a real possibility to have Sicily as a client kingdom of Aragon, if not entierly independent.

Then, assuming James III have himself offsrpings, or that Martin is still alive or have enough childs himself, we could avoid Aragonese Interregnum. Meaning maybe a stronger royal authority above the assemblies, not needed to resolve this conflict (it's always a bad idea asking them to decide who would be king), a lesser Castillan influence on the peninsula.
It could help limiting bandositas, but Peter IV's costly reign made hard avoiding them entierly.

Probably no support for Avignon's pope, or not that certain at least, without the familial relation (The pope being a relative of his wife)

Maybe a more western Meditteranean policy, with Aragon actually taking on outposts and harbours in North Africa. I wouldn't see Sardinia being that succssful in its rebellion, but maybe whoever controls Sicily could take it from himself (that included an Aragonese king of Sicily of course).

So, maybe a stabler, but less hegemonic Aragon?
Very interesting.
 
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