Did he really believe that Juan Carlos I would be an absolute monarch in late 20th century, or did he plan something else entirely, that was prevented by the new Government?
PART 1
Well, you are asking for a tough challenge, that is being able to know what Franco was actually thinking at any point. Knowing his position of power was based in him being the center of the balance amongst several "political famillies" (the army, falangists, traditionalists, monarchists, opus dei...) he was not prone to show his cards regarding key issues like this. Some people will say too that him being galician didn't help to expect clear answers from the guy either, but that's another matter.
Fortunatelly regarding the issue of succession, we can count with an interesting source where Franco himself openly talks about the issue.
Francisco Franco Salgado-Araujo was Francisco Franco Bahamonde's cousin (Franco Bahamonde is the famous one, of course) and the head of Franco's Military House. Thus, he had usual private conversations and confidences with the other Franco, and methodically recorded those conversations in a personal journal that was later published in 1975 entitled as "Mis conversaciones privadas con Franco" (My private conversations with Franco"). It covers conversations from 1954 to 1971. It's a good source to shed some light about Franco's actual ideas for different reasons: They were private conversations with a man trusted by Franco. Franco didn't know his cousin was recording the conversations, and also, the cousin lacked any political or personal ambition doing it, since he never published his notes during his life. The foreword of the recollection is telling about its nature (everything in parenthesis in this quote and the following quotes in mine):
My only goal is to make the absolute truth be known. By someone who lived many years close to El Caudillo, who owes gratitude to him like any other spaniard but never forgets that, above Franco, is the Fatherland (Patria) and the loyalty owed to it.
Amongst the many issues they talked about during the almost two decades the notes record, the matter of the succession was obviously one of them, and a recurrent one in fact.
From the conversations with his cousin we can extract some conclusions:
Franco had a more or less clear idea about his succession from an early moment, even if he reserved it to his more private confidences (I will add a bit of political context below) For him the legitimate successor was Don Juan de Borbón, Alfonso XIII's son and Juan Carlos I father. But Don Juan's political positions (and in Don Juan's case, "positions" in plural is the accurate word) made him an impossible option for Franco. In March 1945, in the so-called "Manifiesto of Laussane", Don Juan, knowing the francoist regime was facing a very delicate and unclear future with the imminent defeat of the Axis powers, and trying to court the favour of the Allied powers, called for a monarchist restoration in Spain. He openly linked the francoist regime with the totatilarian powers.
The regime established by General Franco, since the beginning inspired by the totalitarian Axis powers, is fundamentally incompatible with the current world circumstances (...)so I rise my voice to ask General Franco, aknowledging the failure of his totalitarian conception of the state, to leave the power, allowing free pass to the Restoration (of the monarchy)
Obviously Franco never forgot this declaration. Furthermore, Franco didn't like some of the members in Don Juan's entourage, considering them too liberal or too close to franc-masonry (one of Franco's obssesions) It didn't help that Don Juan played all the strings, keeping contacts with the opposition, the monarchists inside Spain, and of course Franco himself.
So, with Don Juan out of the cards, there were two options left: Don Juan's elder son, Juan Carlos I, and Alfonso de Borbón Dampierre, son of the Infante Don Jaime, Alfonso XIII's second son who had renounced to his rights to the throne due to his deafness (after his older brother, Alfonso, had made the same to get married with a commonner woman)
Anyway, the most legitimated option was Juan Carlos, and it's in the following years when the negotiations between Franco and Don Juan about Juan Carlos future begun. As it happened, Juan Carlos would be educated in Spain under Franco's overseen. Despite Don Juan's will was to give the prince an international education, Franco preferred to avoid external influences and Juan Carlos education was completly held in Spain. Meanwhile Alfonso was considered a backup option in case the other option failed, though the guy even married Franco's grandaughter.
So, going finally to your question, what was Franco's idea about Spain under Juan Carlos? Well, following the aforementioned source, in 1955 he points that
"The new Monarchy will be based over the postulates of Falange"
Later, in 1959, he is more clear:
"a Monarchy based in the principles of the Movimiento ( Movimiento Nacional, the umbrella term to include the forces behind the 1936 uprising and the nature and the only party of the regime, with the short name of Falange Española Tradicionalista y de las Juntas de Ofensiva Nacional Sindicalista (FET-JONS)) not one similar or like the one that fell the 14th of April (14th april 1931 was the proclamation of the Second Republic) because it wouldn't last ever one year and would bring chaos to Spain, making the Crusade useless" (Cruzada Nacional was a term used by the nationalists to refer the Civil War)
It doesn't seem Franco's vision on the issue changed too much in the following years, not certainly in a verifiable way, though some figures of the later years of the regime afirm that during his final years Franco knew that after him deeper changes would be needed. It's not impossible considering the growing internal and external pressure over the regime, but on the other hand in the 70's Franco was an octogenarian, far from the overcontrolling strongman of previous decades, ill and weakened by age, doesn't seem the kind of person who changes his opinions radically or can get a good understanding of the a quickly changing society around him.
However, Franco was many things, but not a gambler. Thus, he kept always an eye over Juan Carlos. In this regard, there is an interesting memo from the Dirección General de Seguridad (The organ charged with wide competences about public order) about a meeting between Juan Carlos and some "progressive" individuals (for the standards of the regime, including politicians, professionals, bankers and bussinesmen) In the meeting, in 1966, the future of Spain after Franco was discused. Juan Carlos hosts considered that the future Spain should have a political system similar to that in other western european countries. In the memo it's said
The Prince suggested that the excesses of pluripartisanism should be avoided. Villar and others added that it would be enough with an inteligent electoral law to grant in the practice a two parties system, social democratic and christian democratic, with some other marginal or complementary sectors"
Still the report points that:
In every moment the Prince talked with respect about the Head of the State and didn't take any commintment.
Of course this report went directly to Franco's desk, though Franco didn't seem worried about it. After all, the Prince had not made any promise and he should have to swear an oath to the principles of the Movement before becoming officially successor.
But also it's important to note that, as powerful as Franco was, he was not the only figure involved in the decissionmaking of the time, and specially from the 60's onwards his role was more that of a refree in the political bickering amongst the diferent factions inside the regime, mainly the strong and mercyless rivalry between Falange and Opus Dei. And in this rivalry, Juan Carlos and the succession was a key front.
Franco's succession wasn't a big worry before the 60's, when the dictator came to a certain age and started to show some health problems. Also, in the 60's the deep changes the spanish society would see in the next two decades after the implementation of the 1959 "Stabilizatation Plan" were becoming visible, and it didn't affect only to daily life of the average spaniard, they also drastically changed the political landscape and brought a new wave of very active opposition against the regime.
But let's go a bit before. The 40's and early 50's had seen a failed attempt on economical Autarky with disastrous consequences that caused Spain to take 20 years to recover the economical levels of 1936. Realizing, by the force of reality and the shadow of a default of the spanish debt, the inviability of this path, and with the 1953 "Madrid Accords" with the USA in the background, Franco more or less reluctantly acceded to a new economical plan based in a push for economical liberalization and industrialization. The plan was rather succesful in the following two decades, though during its first phase it also caused a big surge of unployoment and a large emmigration of spaniards to other (mainly western european) countries, phenomenon that would have also political consequences.