Aragon-Hungary would be the main
European opponent of the Ottomans, if the union held for a long time.
So much depends on if later unions occur (Castilian-Aragonese, Habsburg). On the one hand, all subsequent royal births of OTL will be altered, but at the same time, the Trastamaras and Habsburgs were deliberately placing "bets" generation after generation, to create larger united domains through marriage alliances.
I'd say that while the empire of Charles V was an unlikely result of several lucky breaks, it was still more akin to becoming a blackjack champion or poker champion than a big winner at roulette or your typical lottery, because it was enabled by a degree of preparation and strategy.