There is a uchronia about this issue in the book called La Historia de España que no pudo ser (The history of Spain that could not be), composed of several alternate histories made by historians.
On this particular issue, it counts as during the time of Franco's eldest granddaughter married the Infante Alfonso de Borbón y Dampierre (*), the environment and the family of Franco saw the prince Juan Carlos as too liberal and doubted his intentions towards the Franco's family when he would replace Franco (then he had not produced any evidence around the world that the family of a dictator could stay in their country when this country became a democratic state again). Moreover, Juan Carlos suspected that after the wedding, any time a potentially senile Franco finally accept the claims of his family (his wife and son in law in front). However, Franco maintained his decision primarily for the support of Admiral Carrero Blanco to the election of Juan Carlos and certainly by the fact of not having to admit to everyone that had made a first choice, that he now considered erroneous.
However, the attack that killed Carrero Blanco could be to try more seriously to try and alter the projected Franco succession, as though this attack was carried out by the terrorist group ETA, was widely supported by elements within the state Francoist because Carrero eliminate served many purposes (i.e., many opponents of exile and other internal elements prone to political openness saw him as the guardian of orthodox Francoism that would prevent the Spanish democracy after Franco's death, not knowing that he had promised personally to Prince Juan Carlos would submit his resignation and walk away from politics when Franco died; the United States knew that he was the main proponent of the Spanish nuclear program and was not inclined to re-issue the treaty that allowed the establishment of U.S. bases on Spanish soil; he was one of the few leaders of the Francoism that defending a relentless struggle against corruption, etcetera), among them a great chance to convince Franco to appoint as successor to Alfonso, to the detriment of the already appointed Juan Carlos since 1969.
In addition, during these last years of Franco, while Juan Carlos was positioned within the most likely items to a political opening, Infante Alfonso was positioned in the Falangists sectors what sought to maintain Francoist regime at all costs for editing it to the Falangism's postulates originals (never genuinely accepted by Franco himself) and preventing a possible political opening exploiting existing loopholes that ultimately led to the OTL democratic transition.
Therefore, if Alfonso would become the successor appointed by Franco, he would reign with the name of Alfonso XIV and probably would have given a facelift to the Franco dictatorship to modernize it without losing its authoritarian essences. It produced a similar repression set by Pinochet in Chile since Franco appointed Alfonso as successor, thereby preventing a possible democratization of Spain until the final years of the Cold War when it convinced the Francoist authorities to maintain the dictatorship under these circumstances would be counterproductive to their own short-term interests, imitating the democratic transitions in the countries of Eastern Europe that escaping of the Soviet yoke. Thus, King Alfonso XIV would be forced to stay ahead of the democratization of the country by his abdication in his firstborn Francisco unless he want repeat the fate of his grandfather, overthrowing the Spanish monarchy in 1931.
(*): Remember that Alfonso was the eldest son of the Infante Jaime of Bourbon, the second son of King Alfonso XIII. Jaime, who was deaf after a surgical operation at the age of two years, was forced to abdicate his rights in 1933 after being pressured by his father, who threatened to take their financial support during the exile after the proclamation of the Second Republic and pressed him to marry a noble so their offspring could not be reach a restored Spanish throne because of the Pragmatic Sanction. Juan Carlos is the eldest son of the Infante Juan de Bourbon, the third son of King Alfonso XIII. The first son of King Alfonso XIII, the Infante Alfonso de Borbon y Battemberg, was also pressured to abdicate in 1933 due to his hemophiliac condition and had no descendents.
About the children of Alfonso and Franco's granddaughter, the first son was called Francisco (like his great-grandfather) and he was born in November 1972 (he died in a car accident occurred in OTL 1984). The second and last child was called Luis Alfonso and born in April 1974 (he survived that 1984 car crash and succeeded his father in his dynastic rights to the French throne when his father died in the mysterious "accident" Ski in OTL 1989).
