It wasn't the Pope's idea. It was proposed by Cardinal Wolsey to Cardinal Campeggio, the Papal legate, soon after Campeggio arrived in England in October 1528, sent by the Pope to deal with the "King's Matter."
"They [Wolsey and the English prelates] have thought of marrying the Princess, by dispensation from his Holiness, to the King's natural son, if it can be done. At first I myself had thought of this as a means of establishing the succession, but I do not believe that this design would suffice to satisfy the King's desires."
The Pope's secretary Giovanni Baptista Sanga wrote back:
"With regard to the dispensation for marrying the son to the daughter of the King, if, on the succession being thus established, the King will reject his first thought of the divorce, the Pope will be much more inclined to grant it. I will write more diffusely on the return hither of the cavalier Casale."
But Henry VIII wasn't interested and the matter was dropped.
Sources:
Letters and Papers, Foreign and Domestic, Henry VIII, Volume 4, 1524-1530.
October 1528, entry 4881
https://www.british-history.ac.uk/letters-papers-hen8/vol4/pp2104-2119
Letters and Papers, Foreign and Domestic, Henry VIII, Volume 4, 1524-1530.
December 1528, entry 5072
https://www.british-history.ac.uk/letters-papers-hen8/vol4/pp2208-2254