I also find this would be very hard to achieve. You would need a lot of events coincidentally happen in a way that benefits Portugal and damages everyone else.
Like this: Navarre not only survives, but becomes bigger. Catalonia+Aragon stays independent, or becomes part of France, or dominated by the latter, but without France trying to go further. Castille falls apart into several states who hate each other so much that they never unify and often make wars with each other, weakening them all. At least one Islamic state should also survive.
While Portugal never has a truly bad king, but several good ones, who conquer Galicia and other areas. At the end, they should control at least one third of the peninsula.
And at the end, a Portuguese king manages not only to become head of a Hispanic confederation with Lisbon as a capital, but also makes this rule hereditary.