Main issue I feel with Castille is that the historical outcome does not at all follow game-wise from the way it's setup. Castille has a terrible start, at least for a major power. They start with one ally in Portugal, but Portugal is basically useless. Portugal doesn't have a large army and only a decent navy, and starts the game getting involved with England's disastrous war with France and/or getting involved with defending territory in Africa that it can't defend without Castillian aid. If handled right Portugal can be some help, but especially for the AI it's worse than useless. There's good reason why a popular tactic is still to just eat Portugal.
Diplomacy is really weird in this patch..I've seen Castile get decent allies in earlier patches but they start neutral to everybody and when they have actually improved relations enough every meaningful ally has it's slots filled..like Austria (who is kind of a natural ally to Castile, both because of history and mutual rivals) filling it's diplo slots with free city opms - free cities, that don't help Austria at all and call the Emperor in anyway
There are other things that are so non-sensical it's cringe worthy. Like France and England rivaling Burgundy..the french and the english king were both trying to get Burgundy on their side. And Castile suffers from this bad diplomacy because it doesn't have anyone who starts friendly to them (only Portugal, and Aragon in 1/10 start when they don't rival each other)
In addition to that the Castilian king has -20% improve relations, that's just the icing on the cake
Then there's Aragon. They practically start rivaled. While Aragon is somewhat weaker than Castille it isn't by much and they've got good allies typically. Naples is a good draw, and getting France just demolishes Castille. They don't gain much there. And it's so very pointless if the PU pops up, as now two rivals come together after they've been spending money and manpower killing each other.
France is it's own problem, they start united when they really shouldn't, making them able to blob at a time when they should focus on internal affairs. France should be a bit more HRE-like, weaker on the offence but as strong or even a little stronger when attacked. I think a western Europe DLC is needed that adresses the HRE and France, with minor changes and flavour for Iberia, Italy and England
I agree that Aragon rivaling Castile is bad
Down south you've got foes that aren't up to par for Castille, but more than capable to take advantage if Castille gets weakened, and are usually willing to give it a go. If Castille wins such a fight, they get little as the lands aren't great, can't be converted till later, and are great for rebels. All while costing double the admin. But if they lose they lose territory. Terrible combo, lose/lose.
They've got pretty much the worst ruler setup imaginable. That certainly never helps. Terrible rulers, need to roll the die to get the PU to become like traditional Spain, and they'll spend the first years being backwards in tech advancement because they lack much connection to Italy. Made then worse by a disaster that just sucks. The rewards are terrible and the rebels are harsh.
The starting ruler could really be improved, at least by +1 in each stat
The Castilian civil war should always give Isabella as the Aragonese candidate. First it's weird when you got a female to trigger the wedding (since Aragon often has a male ruler/heir), and the Aragonese candidate is a male as well, so you decide for the Portuguese candidate to marry Aragon later. Had the Portuguese candidate won Castile would have ended up under a Portuguese PU
And second Isabella is a 5/6/3 if I remember the event correctly. Since you have to stay mil focused to not fall behind France early a ruler like Isabella would help catching up with adm/dip
Really right now you got 2 options, fall behind France in mil or fall behind Portugal in adm/dip and thus colonization
