It is my understanding that Mary wanted an heir desperately, and wanted a Catholic husband, hence why she married her cousin Philip. But is there any speculation on what the plan was if they had conceived an heir? Would that child, especially if it was a Son, be both the King of England and of Portugal jointly? That probably would not have gone over well for the English nor the Portuguese correct?
Philip was King of Naples and Sicily (and claimed the title of King of Jerusalem) when he married Mary I in 1554.
He became King of Spain in 1556. After Philip became King of Spain, he and Mary styled themselves: Philip and Mary, by the Grace of God King and Queen of England, Spain, France, Jerusalem, both the Sicilies and Ireland, Defenders of the Faith, Archdukes of Austria, Dukes of Burgundy, Milan and Brabant, Counts of Habsburg, Flanders and Tirol.
(You will notice the claim to the throne of France, but no claim to the throne of Portugal.)
Mary died in 1558. At this time, Philip lost the titles of King of England and King of Ireland which had only been granted to him (under the terms of the "Act of Marriage" which had been passed by the English Parliament) for the duration of his marriage with Mary. He also lost any claims to the throne of France which were derived from Mary's longstanding English claims to the throne of France (though Philip had other, independent claims to the French throne).
At the time of Mary's death, Philip was not King of Portugal. He did not become King of Portugal (King Philip I of Portugal) until 1581.
So, there was never a thought during Philip and Mary's marriage that any child of theirs would be King of Portugal.
If they had had heirs, and their marriage had continued longer, however, their heirs would have been in control of Spain, Italy, the Netherlands, England, Ireland, large parts of the Holy Roman Empire, and an extensive overseas empire, ruling more of Europe than anyone since the Romans, and a considerable portion of the world.
The idea for a Spanish - English union was actually rather established by the time Mary Tudor was married to Philip II. After all, Mary's mother Catherine of Aragon was an aunt of Charles V, Philip's father. She was betrothed to Henry VIII at a time of good relations between the Habsburg and the Tudors. In fact, Catherine was originally married to Arthur the heir to the English throne, but Arthur died. It is believed that Henry VII thought the dowry too large and the alliance too important, such that he arranged for Arthur's younger brother, who will be Henry VIII, to marry Catherine. Later on, when Henry VIII started his turn against the Catholic Church, he asked for annulment of that marriage on the grounds that Catherine and Arthur had consummated their marriage, something Catherine vehemently denied.
As an ally of Charles V, Henry VIII had pursued a marriage between Mary and Charles, however Charles married a Portuguese bride instead. Charles then counter-proposed that Mary be betrothed to his son Philip II.
In this era, the English trade and position was desirable to Charles V and Philip II, as it provided a northern orbit that would connect Spain - England - Low Countries. This was considered such an important strategic concern that Philip II pursued a policy of appeasement even as Elizabeth turned England into Protestantism. I've written a bit about this here in case you are interested.