A Real Life Battle of the Bastards

When King Fernando of Portugal died, there were several claimants to the throne. One was his daughter, Beatriz. But she was married to a foreigner and her mother was extraordinarily unpopular. Another was Pedro I's son, Joao, duque de Valencia de Campos by Pedro's posthumously recognized wife, Inez de Castro. Then there was Pedro's younger son by Inez, Diniz, duque de Cifuentes. And the son that everyone agreed was a bastard, Joao, Master of Aviz, who later became Joao I.

The duque de Valencia had been tricked by Queen Leonor (wife of Fernando) that he would be able to marry his niece (this is Iberia, folks, it's how they roll) Beatriz, if he (Valencia) were just to get rid of that slight problem called his wife. The duchess was murdered, and Valencia's popularity plummeted, and he found out that Queen Leonor had been lying and that she never intended for him to marry his niece at all. Valencia and his brother were both married to Castilian bastard daughters of Enrique II.

When it came down to is, the Master of Aviz walked off with the crown of Portugal. But what if Valencia's popularity hadn't plummeted (either he doesn't fall for Leonor's talk and have wifey murdered or he somehow redeems himself thereafter). As the "unquestionably" legitimate son if Pedro's marriage to Inez indeed ever took place (apparently there's no contemporary evidence for it beyond Pedro's assertion that it had happened), Valencia would (in theory) be in a better position than a woman (wed to a foreigner) or a bastard (as I said, there were no doubts about the Master of Aviz's lack of legitimacy). The only contender would still be brother, Diniz.

Now, of course, if the duque de Valencia were to just become king of Portugal that would just be plain boring. So, what if it ends up like GoT's Battle of the Bastards? Aviz gets part of the population to support him, and Valencia has the support of the remainder. The civil war comes to a halt somewhere, and the two claimants duke it out. Aviz ends up dead or imprisoned after the battle (awaiting execution a la Ramsey Bolton).
 
Well the Battle of the Bastards in the TV show is as inaccurate as most TV portrayals of medieval combat but a Portuguese expy of the Wars of the Roses complete with Towton and Bosworth equivalents is perfectly plausible.
 

Lusitania

Donor
Ok to those not familiar with this part of Portuguese history we are dealing with 3 different johns.

John of Valencia - was legitimate son of Portuguese king Pedro and younger half brother of king Fernando. He had substantial holding in Portugal but due to death threats fled to Castile where he became known as John of Valencia.

John of Avis - illegitimate son of king Pedro who while liked was not as well liked as his brother John of Valencia. Due to John of Valencia being held in prison by the king of Castile the Portuguese royalty and people wishing to stay independent of Castile rallied around John of Avis to fight King if Castile.

John I of Castile - married to infanta Beatrix the only child of Portuguese king Ferdinando. It was through his marriage to Beatrix that king of Castile has the claim to Portuguese thrown (although he also has some claim of his own). (Remember the royals of the Iberian Peninsula were the most inbred royals in Europe).

Now for your premise. John of Valencia as the “Legitimate” son of king Pedro John had overwhelming public and noble support plus was married to very powerful Portuguese noble family. He was the favorite son of his dad and had very good relations his brother.

Prior to the war of succession 1383-1385 he had fled Portugal to avoid the family “Mendes” of his murdered wife. He fled to Castile and when the crises of succession occurred John was still very popular with certain segments of population and nobles which scared king John I of Castile só king of Castile had him imprisoned.

After war his relationship with half brother John I of Portugal was not good and John stayed in Castile.

So if his reputation never suffered he would in fact be representing Portuguese claims and not his half brother John of Avis. He would be leading Portuguese against Castile.

To get the war of Bastards as you say we need to illiminate threat of Castile. As long as that existed the people of Portugal would of rallied around the strongest candidate for the throne.
 
Top