In 1682 the only heiress of the Portuguese throne was the princess Isabel Luisa, daughter of the regent (later king) Peter II of Portugal and Marie Françoise of Savoy. Her mother, who was pro-French and had the support of Louis XIV, tried to marry Isabel to her cousin the Duke of Savoy, Victor Amadeus II. However, when the comitive which should take him to Portugal arrived the refused to go, pretending to be ill. It's believed that he was pressured in court to not to go, because his advisors were affraid that Louis XIV wanted to make him the king consort of Portugal as a compensation for a future French annexation of Savoy. Also, Isabel was considered of fragile health, a reputation that didn't help her.
Later, in 1683, Marie Françoise died and the king married in 1687 Maria Sophia of Neuburg, who gave him many other heirs, including the future John V of Portugal. Isabel died unmarried in 1690.
But WI both Marie Françoise and Isabel were more healthy, living longer and then not allowing Peter II to remarry? Also, if Isabel remains as the only heir of Portugal and Victor Amadeus is convinced to marry her, what could happen? If they have a male son we would have a Portuguese-Savoyard personal union in the future. But would Louis XIV try to take all Savoy to France? And would it have some influence in the future wars against the French (the Nine Years War and the Spanish Succession War)?
I know the POD is a bit complex, but I was really interested in the consequences of such union.