Maybe if Charles of Viana and Agnes of Cleves produce an heir a united Basque and Aragon would be able to take advantage of colonization. You would also need to somehow avoid the http://en.wikipedia.org/wiki/War_against_John_II or make sure it is fast and victorious. I think the major reason why Portugal was successful, was because of their stable monarchy. The Aragonese however were often at war with themselves, and always caught between France, Castille, and the Italian situation making the nation very unstable.
I actually had something like that happen in my Palaiologos TL. Charles of Viana won the civil war with help from King Louis XI of France, but since he was already middle aged without a legitimate son I had him promote his illegitimate son Philip (he had a deal with Louis XI and an antipope that allowed him to legitimize his son). Unfortunately they then lost control of Sicily (and southern Italy wasn't theirs at my PoD), but that gives them some incentive to look west I think.