Portuguese history has several would-be queens.
First was Infanta D.Beatriz that some even call Queen Beatriz (1373-1412/20 ?)
She was the only daughter of King D.Fernando and Queen Leonor Telles.
When her father died, in 1383, she found herself as her sole living heiress as her two younger brothers had predecesed the king.
The king had thought of marrying her 10 years daughter with a younger son of the king of Castille, but suddenly decided to marry her to the old widower king Juan I (1358-90).
A treaty was celebrated under which neither Beatriz nor Juan would be Queen or King of Portugal, but the throne would be reserved for an hypotetical son of the couple who, when he would be 3 months old, would be brought to Portugal and raised as portuguese.
Two monts later, D.Fernando died and Juan I, in agreement with his mother-in-law, invaded Portugal and proclaimed himself and his wife King and Queen of Portugal.
They were never recognized as such by most of the nobilty, the clergy and the merchants, there was a two years war, known as the Portuguese Interregnum, which saw victorious D.João I, half-brother of the late D.Fernando, who started the Aviz dynasty.
Another would-be queen regnant was D.Catarina of Bragança.
During the reign of King D.Manuel I, Portugal was at his peak, in prestige, wealth and the royal family was rather numerous.
Hard to imagine that in 3 generations it would be extinct in the male line.
D.Manuel was succeeded by his son D.João III whose 9 children predecesed him.
His second son D.Luis, duke of Beja had an illegitimate son, D.António
Third and fourth sons D.Afonso and D.Henrique were cardinals.
Fifth son, D.Duarte, duke of Guimarães had a son (w/o succession) and two daughters - the eldest, D.Maria, duchess of Parma by marriage and the youngest D.Catarina, married her cousin the duke of Bragança and were the ancestors of the present house of Bragança.
When the last Aviz died in 1580, cardinal D.Henrique, there were several pretendantes: D.António, who claimed his father had legitimized him, had the favour of the lower classes but was despised by aristocracy and the clergy, and specially the RF, D.Henrique and D.Catarina.
Then came Felipe II as the elder son of D.Manuel's elder daughter, and D.Catarina as the only portuguese living daughter of a younger son - with a better claim than Felipe II.
But Felipe, as he wrote, bribed and conquered and won the duchess pretensions with promises, some never fulfilled, and the Braganças had to wait 60 years to have their rights recognized.
I know that royal history is replete with examples of male persons who would have become kings, had they lived -- whether or not they were born direct in successions or became crown princes. It's entirely possible for someone originally not close or direct in line to the throne to find himself in a position where factors and forces move him toward it. Even if he was never the heir apparent, hindsight tells us that only his death prevented him from succeeding.
This varied group includes the likes of Infante Miguel de la Paz of Portugal (1498-1500), grandson of the famous Catholics monarchs of Spain, who would have united the Iberian peninsula under one rule had he lived; Henry Frederick, Prince of Wales (1594-1612), eldest son of King James VI/I of Scotland and England; the two dauphins of France under the reign of the famous "Sun" King Louis XIV; Prince William, Duke of Gloucester (1689-1700), whose death prompted Britain's parliament to pass the fateful Act of Settlement in 1701; Prince Frederick of the United Kingdom, Duke of York (1763-1827), second son of King George III, Prince Edward of the United Kingdom, Duke of Kent (1767-1820), fourth son of King George III and father of Queen Victoria; and Prince Luitpold of Bavaria (1821-1912), who served as regent of the kingdom for the last 26 years of his life, during which time he was heir presumptive to the throne.
Prince Knud of Denmark (1900-1976) obviously doesn't count, for it was a constitutional amendment changing the law that prevented him from sitting on the throne -- not survival (he had survived his older brother, King Frederik IX, by four years).
Anyhow, bearing all this in mind, I was wondering about a list of royal females who would have succeeded as queens regnant, had they lived. There are several coming to mind, off the top of my head: Infanta Isabel (1470-1498), mother of Infante Miguel de la Paz, who became the queen of Portugal and who would have become the queen of Spain had she lived; Mary, Queen of Scots, who might have succeeded Queen Elizabeth I on the English throne, had she not been so misguided in her actions, eventually leading her to the block and the axe; the Electress Sophia of Hanover, who was heiress presumptive to Queen Anne on the British throne for the last 12 years of her life; Princess Charlotte of Wales (1796-1817), whose tragic death in childbirth prompted several uncles to cast aside their mistresses and marry princesses, in the race to produce heirs to the British throne; and Princess Elizabeth Georgina of Clarence (1820-1821), daughter of the future King William IV, hailed at her christening as "Little Queen Bess".
Can anybody name additional examples? Infanta Maria de las Mercedes of Spain (1880-1904) obviously doesn't count, despite the fact that she was heiress presumptive to the throne (first in the line of succession, no less) until her dying day. Her only brother )King Alfonso XIII) although unmarried at the time, would eventually marry Princess Ena of Battenberg and father multiple children by her. So one can't say that only death prevented her succession.